igc: use EOPNOTSUPP instead of ENOTSUPP in igc_ethtool_get_sset_count()

igc_ethtool_get_sset_count() returns -ENOTSUPP when a given stringset is
not supported, causing userland programs to get "Unknown error 524".

Since EOPNOTSUPP should be used when error is propagated to userland,
return -EOPNOTSUPP instead of -ENOTSUPP.

Fixes: 36b9fea ("igc: Add support for statistics")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: hyperenju

drivers/net/ethernet/intel/igc/igc_ethtool.c

@@ -810,7 +810,7 @@ static int igc_ethtool_get_sset_count(struct net_device *netdev, int sset)
 	case ETH_SS_PRIV_FLAGS:
 		return IGC_PRIV_FLAGS_STR_LEN;
 	default:
-		return -ENOTSUPP;
+		return -EOPNOTSUPP;
 	}
 }