mt76: use udelay instead of usleep_range in mt76x2_mac_stop

usleep_range can cause excessive latency on channel change if waiting
for the MAC to stop fails. It will be forced to stop by the code
following that loop anyway.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>