mt76: move mt76x02_sw_scan and mt76x02_sw_scan_complete in mt76x02-lib module

Move mt76x02_sw_scan and mt76x02_sw_scan_complete utility routines
in mt76x02_util.c in order to be reused by mt76x0 and mt76x2u drivers
and remove duplicated code

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