Fixes a crash on MMIO devices when running into the watchdog reset

Fixes: d3377b78 ("mt76: add HE phy modes and hardware queue")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200615181341.81871-1-nbd@nbd.name