staging/rtl8712 : Remove unused macro definitions.

WLAN_ETHHDR_LEN/WLAN_ETHADDR_LEN/WLAN_ADDR_LEN
 are unused, duplicate or unnecessary, remove.

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