staging: rtl8723au: pxmitframe->frame_tag is never set to a value > 8

No point of masking out high bits since we never set the value to
anything exceeding bits 0-3.

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