sfc: Remove duplicate argument

Fix the following coccicheck warning:

./drivers/net/ethernet/sfc/enum.h:80:7-28: duplicated argument to |
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/enum.h

@@ -78,7 +78,6 @@ enum efx_loopback_mode {
 			    (1 << LOOPBACK_XAUI) |		\
 			    (1 << LOOPBACK_GMII) |		\
 			    (1 << LOOPBACK_SGMII) |		\
-			    (1 << LOOPBACK_SGMII) |		\
 			    (1 << LOOPBACK_XGBR) |		\
 			    (1 << LOOPBACK_XFI) |		\
 			    (1 << LOOPBACK_XAUI_FAR) |		\