staging: rtl8188eu: use is_broadcast_ether_addr

Use is_broadcast_ether_addr instead of checking each byte of the
address array for 0xff. Shortens the code and improves readability.

As required by is_broadcast_ether_addr, the address array sta_addr
is properly aligned in all uses. Thanks to Joe Perches.

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