igb: e1000_82575: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/net/ethernet/intel/igb/e1000_82575.c: In function ‘igb_get_invariants_82575’:
drivers/net/ethernet/intel/igb/e1000_82575.c:636:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

   if (igb_sgmii_uses_mdio_82575(hw)) {
      ^

drivers/net/ethernet/intel/igb/e1000_82575.c:642:2: note: here
case E1000_CTRL_EXT_LINK_MODE_PCIE_SERDES:
^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable -Wimplicit-fallthrough.

---
 drivers/net/ethernet/intel/igb/e1000_82575.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index bafdcf70a353..3ec2ce0725d5 100644
@@ -638,7 +638,7 @@ static s32 igb_get_invariants_82575(struct e1000_hw *hw)
 			dev_spec->sgmii_active = true;
 			break;
 		}
-		/* fall through for I2C based SGMII */
+		/* fall through - for I2C based SGMII */
 	case E1000_CTRL_EXT_LINK_MODE_PCIE_SERDES:
 		/* read media type from SFP EEPROM */
 		ret_val = igb_set_sfp_media_type_82575(hw);

2.20.1