mt76: move mt76x02_rx_get_sta and mt76x02_rx_get_sta_wcid in mt76x02_util.h

Move mt76x02_rx_get_sta and mt76x02_rx_get_sta_wcid utility routines in
mt76x02-lib module since it will be used by mt76x0 driver in order to
unify rxwi parsing

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