mwifiex: make use of is_zero_ether_addr()

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

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