rt2x00: Implement get_antenna and set_antenna callback functions

Implement the get_antenna and set_antenna callback functions, which will
allow clients to control the antenna for all non-11n hardware (Antenna handling
in rt2800 is still a bit magical, so we can't use the set_antenna for those drivers
yet).

To best support the set_antenna callback some modifications are needed in the
diversity handling. We should never look at the default antenna settings to determine
if software diversity is enabled. Instead we should set the diversity flag when
possible, which will allow the link_tuner to automatically pick up the tuning.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>