rt2x00: Fix max TX power settings

During initialization each driver reads the default TX power
for each individual channel. However mac80211 only accepts the
maximum value (which is also handled as default value).

As a result, the TX power of the device was being limited to
the default value, which is often quite low compared to the
real maximum acceptable value.

This patch allows each driver to set the maximum value on a
per-channel basis which is forwarded to mac80211. The default
value will be preserved for now, in case we want to update
mac80211 to differentiate between the maximum and default txpower.

This fixes bug complaining about limited TX power values like:
https://bugzilla.kernel.org/show_bug.cgi?id=16358



Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>