mt76x2: add mac80211 {set,get}_antenna callbacks

Add capability to select tx/rx antennas. Possible values are:
 - 1: to use only the first antenna
 - 2: to use only the second antenna
 - 3: to use both of them

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