staging/wlan-ng : Remove duplicated defines from p80211

ETH_ALEN/ETH_HLEN can be  used instead of
WLAN_ETHADDR_LEN & WLAN_ETHHDR_LEN, replace directly or
use eth_addr* functions where applicable.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>