staging: rtl8188eu: use is_multicast_ether_addr in rtw_sta_mgt.c

Use is_multicast_ether_addr instead of custom IS_MCAST in
core/rtw_sta_mgt.c.

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