mt76x0: remove mt76x0_stop_hardware routine

Since it is actually used in a single place and it just
runs mt76x0_chip_onoff routine, remove mt76x0_stop_hardware
and use mt76x0_chip_onoff directly

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