mt76: introduce mt76_{incr,decr} utility routines

Add mt76_{incr,decr} utility routines to increment/decrement a value
with wrap-around (they will be used by mt76x2 DFS sw detector)

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