mt76: move mt76_reg_pair definition in mt76.h

Move common definition of mt76_reg_pair in mt76.h
and remove duplicated code in mt76x2_init_common.c

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