mt76x2: make mt76x2_mac_reset routine static

Add static qualifier to mt76x2_mac_reset routine and remove the
prototype from mt76x2_mac.h since it is used just in mt76x2_init.c

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