rtw88: fix potential NULL pointer access for firmware

Driver could access a NULL firmware pointer if we don't
return here.

Fixes: 5195b904

 ("rtw88: avoid FW info flood")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>