staging: r8712u: use is_broadcast_ether_addr() to simplify the code

Using is_broadcast_ether_addr() to simplify the code.

spatch with a semantic match is used to found this problem.
(http://coccinelle.lip6.fr/

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>