igb: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements instead of just
letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igb/e1000_phy.c

@@ -836,6 +836,7 @@ s32 igb_copper_link_setup_igp(struct e1000_hw *hw)
 			break;
 		case e1000_ms_auto:
 			data &= ~CR_1000T_MS_ENABLE;
+			break;
 		default:
 			break;
 		}

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

@@ -3010,6 +3010,7 @@ static int igb_set_rxnfc(struct net_device *dev, struct ethtool_rxnfc *cmd)
 		break;
 	case ETHTOOL_SRXCLSRLDEL:
 		ret = igb_del_ethtool_nfc_entry(adapter, cmd);
+		break;
 	default:
 		break;
 	}

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

@@ -1008,6 +1008,7 @@ static int igb_ptp_set_timestamp_mode(struct igb_adapter *adapter,
 	switch (config->tx_type) {
 	case HWTSTAMP_TX_OFF:
 		tsync_tx_ctl = 0;
+		break;
 	case HWTSTAMP_TX_ON:
 		break;
 	default: