mt76: mt7921: add missing bh-disable around rx napi schedule

napi_schedule() can call __raise_softirq_irqoff(), which can perform
softirq handling, so it must not be called in a pure process context
with BH enabled.

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