mt76: initialize available_antennas_{tx,rx} info

Initialize available_antennas related info in wiphy data structure
according to antenna_mask field; antenna_mask info is initialized
in device specific code and will be used in mac80211 {set,get}_antenna
callbacks

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>