wifi: mt76: dma: do not increment queue head if mt76_dma_add_buf fails

Do not increment queue head if mt76_dma_add_buf fails for Wireless
Ethernet Dispatcher rx queues.

Fixes: cd372b8c ("wifi: mt76: add WED RX support to mt76_dma_{add,get}_buf")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>