staging: rtl8723au: Fix resource leak

Firmware was not released properly if kmemdup fails.

Addresses-Coverity-Id: 1269118
Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>