rtlwifi: Add code to read new versions of firmware

Changes in the drivers for RTL8723BE and RTL8821AE require corresponding
changes in the firmware. This new firmware has been accepted into the
Linux firmware repo. To handle the case where the kernel has been
updated before the firmware, the new versions have been given new names.
The code will attempt to read the new name, and fall back to the old one
if the new one is not available.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>