Add constant for FCS/CRC length (frame check sequence)

About a dozen drivers that have some form of crc checksumming or offloading
use this constant, warranting a global define for it.
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

include/linux/if_ether.h

@@ -33,6 +33,7 @@
 #define ETH_ZLEN	60		/* Min. octets in frame sans FCS */
 #define ETH_DATA_LEN	1500		/* Max. octets in payload	 */
 #define ETH_FRAME_LEN	1514		/* Max. octets in frame sans FCS */
+#define ETH_FCS_LEN	4		/* Octets in the FCS		 */
 
 /*
  *	These are the defined Ethernet Protocol ID's.