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

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

BugLink: https://bugs.launchpad.net/bugs/1938340
[ Upstream commit 8d3cdc1bbb1d355f0ebef973175ae5fd74286feb ]

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: 6aa4ed7927f11 ("mt76: mt7615: implement DMA support for MT7622")
Fixes: 4bb586bc33b98 ("mt76: mt7663u: sync probe sampling with rate configuration")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>