Currently if the allocation of skb fails and returns NULL then the
call to skb_put will cause a null pointer dereference. Fix this by
checking for a null skb and returning NULL.  Note that calls to
function mt76x2_mcu_msg_alloc don't directly check the null return
but instead pass the NULL pointer to mt76x2_mcu_msg_send which
checks for the NULL and returns ENOMEM in this case.

Detected by CoverityScan, CID#14626246 ("Dereference null return value")

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>