staging: rtl8723au: Simplify retrieving the ether_type in portctrl()

This renders the last element in include/ethernet.h unnecessary, so
remove it as well.

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