wifi: mt76: connac: check for null before dereferencing

The wcid can be NULL. It should be checked for validity before
dereferencing it to avoid crash.

Fixes: 098428c4 ("wifi: mt76: connac: set correct muar_idx for mt799x chipsets")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>