rndis_wlan: use is_zero_ether_addr() and is_broadcast_ether_addr()

Use is_zero_ether_addr() and is_broadcast_ether_addr() instead of
memcmp against ffff_bssid/zero_bssid.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>