b43: Only select allowed TX and RX antennas

This fixes antenna selection in b43. It adds a sanity check
for the antenna numbers we get from mac80211.

This patch depends on
ssb: Fix extraction of values from SPROM

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>