staging: r8188eu: use is_multicast_ether_addr in core/rtw_security.c

Use is_multicast_ether_addr instead of custom macro IS_MCAST, all
buffers are properly aligned.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210823120106.9633-6-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>