staging: rtl8723au: validate_recv_data_frame() use fctl knowledge to obtain bssid

Use the knowledge we already have from parsing the TODS/FROMDS bits in
hdr->frame_control to obtain the bssid.

Note that get_hdr_bssid() would never return NULL as handling 4
combinations of a 2 bit word leaves little space for falling through
to the 'default' value.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>