mt76: mt7615: fix NULL pointer dereference in tx_prepare_skb()

Fix theoretical NULL pointer dereference in mt7615_tx_prepare_skb and
mt7663_usb_sdio_tx_prepare_skb routines. This issue has been identified
by code analysis.

Fixes: 6aa4ed79 ("mt76: mt7615: implement DMA support for MT7622")
Fixes: 4bb586bc ("mt76: mt7663u: sync probe sampling with rate configuration")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>