mt76x2: move utility routines in mt76x2.h

In order to reuse them supporting mt76x2u based devices, move
mt76x2_wait_for_mac, wait_for_wpdma and mt76x2_channel_silent in
mt76x2.h

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