wifi: rtw89: use schedule_work to request firmware

Since we are going to load more than one firmware and some are not
presented or optional, using asynchronous API request_firmware_nowait()
will become complicated. Also, we want to use firmware_request_nowarn()
to avoid warning messages when loading optional files. So, use
schedule_work to be simpler.

To abstract loading a firmware or file, define a struct rtw89_fw_req_info
containing a struct firmware and a completion to ensure this firmware is
loaded completely.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230320130606.20777-3-pkshih@realtek.com