rtlwifi: use eth_zero_addr() to assign zero address

Using eth_zero_addr() to assign zero address insetad of
memset() or an inefficient copy from a static array.

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