igb_main: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 200521 ("Missing break in switch")
Addresses-Coverity-ID: 114797 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -5176,6 +5176,7 @@ bool igb_has_link(struct igb_adapter *adapter)
 	case e1000_media_type_copper:
 		if (!hw->mac.get_link_status)
 			return true;
+		/* fall through */
 	case e1000_media_type_internal_serdes:
 		hw->mac.ops.check_for_link(hw);
 		link_active = !hw->mac.get_link_status;
@@ -5836,6 +5837,7 @@ csum_failed:
 			type_tucmd = E1000_ADVTXD_TUCMD_L4T_SCTP;
 			break;
 		}
+		/* fall through */
 	default:
 		skb_checksum_help(skb);
 		goto csum_failed;