staging: vt6656: replaced U_CRC_LEN with ETH_FCS_LEN

Removed custom macro with one from include/linux/if_ether.h.
FCS is an Ethernet field that holds the CRC value.

Signed-off-by: Andres More <more.andres@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>