mt76: make mt76_sar_capa static

Fix the following sparse warning:
drivers/net/wireless/mediatek/mt76/mac80211.c:183:32:
	warning: symbol 'mt76_sar_capa' was not declared. Should it be static?

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>