staging: rtl8188eu: use is_multicast_ether_addr in rtl8188eu_xmit.c

Use is_multicast_ether_addr instead of custom IS_MCAST in
hal/rtl8188eu_xmit.c.

There is only one use, so remove the extra variable for the
result of IS_MCAST.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>