wifi: mt76: do not increase mcu skb refcount if retry is not supported

If mcu_skb_prepare_msg is not implemented, incrementing skb refcount does not
work for mcu message retry. In some cases (e.g. on SDIO), shared skbs can trigger
a BUG_ON, crashing the system.
Fix this by only incrementing refcount if retry is actually supported.

Fixes: 3688c18b ("wifi: mt76: mt7915: retry mcu messages")
Closes: https://lore.kernel.org/r/d907b13a-f8be-4cb8-a0bb-560a21278041@notapiano/


Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com> #KernelCI
Tested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240917110942.22077-1-nbd@nbd.name