staging: brcm80211: Convert ETHER_ADDR_LEN to ETH_ALEN

Add an #include of <linux/if_ether.h> to include/proto/ethernet.h
Convert ETHER_ADDR_LEN, remove the #define for ETHER_ADDR_LEN.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>