mt76: add mt76_init_stream_cap routine

Add mt76_init_stream_cap utility routine to set ht/vht sta capabilities
related to number of streams currently used by the device
(it will be used by mac80211 set_antenna callback)

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