wifi: rtw88: unlock on error path in rtw_ops_add_interface()

Call mutex_unlock(&rtwdev->mutex); before returning on this error path.

Fixes: f0e741e4 ("wifi: rtw88: add bitmap for dynamic port settings")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/ddd10a74-5982-4f65-8c59-c1cca558d239@kili.mountain