cfg80211: Separate available antennas for RX and TX

As has been pointed out by Daniel Halperin some devices (e.g. Intel IWL5100)
can only TX from a subset of RX antennas, so use separate availability masks
for RX and TX.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>