staging: rtl8723au: Request correct firmware file for A-cut parts

The filename requested by the driver didn't match what we had sitting
in /lib/firmware/

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>