staging: rtl8188eu: use is_multicast_ether_addr in rtw_xmit.c

Use is_multicast_ether_addr instead of custom IS_MCAST in
core/rtw_xmit.c. In all uses the address array is properly
aligned.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>