mt76: move mt76x02_mac_work routine in mt76x02-lib module

Move mt76x02_mac_work routine in mt76x02_mac.c in order to be reused by
mt76x0 driver to read device statistics

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