staging: rtl8188eu: use is_multicast_ether_addr

Use is_multicast_ether_addr instead of custom IS_MCAST.
The variable for the result of IS_MCAST was only used in the
if conditional. So remove the extra variable and move the call
to is_multicast_ether_addr into the conditional.

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