Staging: wlan-ng: Remove use of WLAN_ADDR_LEN

Replace the driver local WLAN_ADDR_LEN constant through the kernel-wide ETH_ALEN definiton.

Signed-off-by: Moritz Muehlenhoff <jmm@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>