mwifiex: use is_zero_ether_addr() instead of memcmp()

Using is_zero_ether_addr() instead of directly use
memcmp() to determine if the ethernet address is all
zeros.

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: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>