mt76: dma: add skb check for dummy pointer

Introduce skb check for dummy address in mt76_dma_tx_cleanup_idx.
This is a preliminary patch to add support for new chipsets (e.g. 7615)

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