mt76: do not report out-of-range rx nss

Take into account device rx stream in mt76x02_mac_process_rate
in order to not report wrong number of rx spatial stream to mac80211.
The issue has been reported on mt76x0 device which is 1x1:1 however
the hw sometimes reports rx nss equal to 2

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>