Staging: vt6656: use ETH_HLEN macro instead of custom one

Replace custom header length definition U_HEADER_LEN by ETH_HLEN
from <linux/if_ether.h>. Also remove unused U_TYPE_LEN.

Signed-off-by: Charles Clément <caratorn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>