Allow setting of TX and RX antennas configuration via nl80211.

The antenna configuration is defined as a bitmap of allowed antennas to use.
This API can be used to mask out antennas which are not attached or should not
be used for other reasons like regulatory concerns or special setups.

Separate bitmaps are used for RX and TX to allow configuring different antennas
for receiving and transmitting. Each bitmap is 32 bit long, each bit
representing one antenna, starting with antenna 1 at the first bit. If an
antenna bit is set, this means the driver is allowed to use this antenna for RX
or TX respectively; if the bit is not set the hardware is not allowed to use
this antenna.

Using bitmaps has the benefit of allowing for a flexible configuration
interface which can support many different configurations and which can be used
for 802.11n as well as non-802.11n devices. Instead of relying on some hardware
specific assumptions, drivers can use this information to know which antennas
are actually attached to the system and derive their capabilities based on
that.

802.11n devices should enable or disable chains, based on which antennas are
present (If all antennas belonging to a particular chain are disabled, the
entire chain should be disabled). HT capabilities (like STBC, TX Beamforming,
Antenna selection) should be calculated based on the available chains after
applying the antenna masks. Should a 802.11n device have diversity antennas
attached to one of their chains, diversity can be enabled or disabled based on
the antenna information.

Non-802.11n drivers can use the antenna masks to select RX and TX antennas and
to enable or disable antenna diversity.

While covering chainmasks for 802.11n and the standard "legacy" modes "fixed
antenna 1", "fixed antenna 2" and "diversity" this API also allows more rare,
but useful configurations as follows:

1) Send on antenna 1, receive on antenna 2 (or vice versa). This can be used to
have a low gain antenna for TX in order to keep within the regulatory
constraints and a high gain antenna for RX in order to receive weaker signals
("speak softly, but listen harder"). This can be useful for building long-shot
outdoor links. Another usage of this setup is having a low-noise pre-amplifier
on antenna 1 and a power amplifier on the other antenna. This way transmit
noise is mostly kept out of the low noise receive channel.
(This would be bitmaps: tx 1 rx 2).

2) Another similar setup is: Use RX diversity on both antennas, but always send
on antenna 1. Again that would allow us to benefit from a higher gain RX
antenna, while staying within the legal limits.
(This would be: tx 0 rx 3).

3) And finally there can be special experimental setups in research and
development even with pre 802.11n hardware where more than 2 antennas are
available. It's good to keep the API simple, yet flexible.

Signed-off-by: Bruno Randolf <br1@einfach.org>

--
v7:	Made bitmasks 32 bit wide and rebased to latest wireless-testing.
Signed-off-by: John W. Linville <linville@tuxdriver.com>