wireless, at76c50x:: Don't needlessly test for NULL before calling release_firmware()

The release_firmware() function deals gracefully with being passed a
NULL pointer, so explicit tests before the call are rather pointless.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>