mt76: move mt76x02_beacon_offset in mt76x02_util.c

Move mt76x02_beacon_offset utility routine in mt76x02-lib module
since it is shared between mt76x0 and mt76x2 driver and remove
duplicated code. Moreover move beacon_offset data structure in
mt76x02-lib module since it is shared between mt76x0 and mt76x2
drivers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>