Staging: vt6655: use ETH_FCS_LEN macro instead of custom one

Replace custom FCS/CRC length definition U_CRC_LEN by ETH_FCS_LEN
from <linux/if_ether.h>.

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