rtlwifi: Fix fallback firmware loading

Commit f70e4df2 ("rtlwifi: Add code to read new versions of
firmware") added code to load an old firmware file if the new one is
not available.  Unfortunately that code is never reached because
request_firmware_nowait() does not wait for the firmware to show up
and returns 0 even if the file is not there.

Use the existing fallback mechanism introduced by commit 62009b7f
("rtlwifi: rtl8192cu: Add new firmware") instead.

Fixes: f70e4df2 ("rtlwifi: Add code to read new versions of firmware")
Cc: stable@vger.kernel.org
Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>