rtw88: load wowlan firmware if wowlan is supported

Driver used to download normal firmware only,
but some devices support wowlan and require to
download wowlan firmware when system suspends.
So modify rtw_load_firmware() and its callback to
allow driver to download both normal and wowlan firmware.

Signed-off-by: Chin-Yen Lee <timlee@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>