mt76x0: usb: stop cal/mac workqueues at hw stop

Stop mac and calibration work stopping the hw even if the
device has been removed

Fixes: b11e1969 ("mt76x0: add ieee80211_ops ops pointer to
mt76x0_alloc_device signature")

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