mwifiex: use is_broadcast_ether_addr() instead of memcmp()

Using is_broadcast_ether_addr() instead of directly use
memcmp() to determine if the ethernet address is broadcast
address.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>