wifi: mt76: dma: free rx_head in mt76_dma_rx_cleanup

[ Upstream commit 1b88b47e898edef0e56e3a2f4e49f052a136153d ]

Free rx_head skb in mt76_dma_rx_cleanup routine in order to avoid
possible memory leak at module unload.

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