mt76: add default implementation for mt76_sw_scan/mt76_sw_scan_complete

Introduce a default implementation for mt76_sw_scan and
mt76_sw_scan_complete in mt76 module and remove duplicated code
since most of the drivers share the same implementation

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>