e1000g: this statement may fall through

The gcc 7 does check for switch statement fall through cases, and if legit,
such complaint can besilenced by /* FALLTHROUGH */ comment. Unfortunately
such comment is quite limited, but will still notify the reader.

This patch is backport from illumos, see
https://www.illumos.org/rb/r/941/

Reviewed by:	eadler
Differential Revision:	https://reviews.freebsd.org/D14663

sys/dev/e1000/e1000_82575.c

@@ -1679,7 +1679,7 @@ static s32 e1000_setup_serdes_link_82575(struct e1000_hw *hw)
 	case E1000_CTRL_EXT_LINK_MODE_1000BASE_KX:
 		/* disable PCS autoneg and support parallel detect only */
 		pcs_autoneg = FALSE;
-		/* fall through to default case */
+		/* FALLTHROUGH */
 	default:
 		if (hw->mac.type == e1000_82575 ||
 		    hw->mac.type == e1000_82576) {
@@ -1806,6 +1806,7 @@ static s32 e1000_get_media_type_82575(struct e1000_hw *hw)
 			break;
 		}
 		/* fall through for I2C based SGMII */
+		/* FALLTHROUGH */
 	case E1000_CTRL_EXT_LINK_MODE_PCIE_SERDES:
 		/* read media type from SFP EEPROM */
 		ret_val = e1000_set_sfp_media_type_82575(hw);

sys/dev/e1000/e1000_mbx.c

@@ -779,6 +779,7 @@ s32 e1000_init_mbx_params_pf(struct e1000_hw *hw)
 		mbx->stats.reqs = 0;
 		mbx->stats.acks = 0;
 		mbx->stats.rsts = 0;
+		/* FALLTHROUGH */
 	default:
 		return E1000_SUCCESS;
 	}

sys/dev/e1000/e1000_phy.c

@@ -1298,6 +1298,7 @@ s32 e1000_copper_link_setup_m88_gen2(struct e1000_hw *hw)
 			phy_data |= M88E1000_PSCR_AUTO_X_1000T;
 			break;
 		}
+		/* FALLTHROUGH */
 	case 0:
 	default:
 		phy_data |= M88E1000_PSCR_AUTO_X_MODE;

sys/dev/e1000/e1000_vf.c

@@ -488,8 +488,10 @@ s32 e1000_promisc_set_vf(struct e1000_hw *hw, enum e1000_promisc_type type)
 		break;
 	case e1000_promisc_enabled:
 		msgbuf |= E1000_VF_SET_PROMISC_MULTICAST;
+		/* FALLTHROUGH */
 	case e1000_promisc_unicast:
 		msgbuf |= E1000_VF_SET_PROMISC_UNICAST;
+		/* FALLTHROUGH */
 	case e1000_promisc_disabled:
 		break;
 	default: