Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/drivers/aspeed/common/ast_main.c

@@ -212,7 +212,7 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 					ast->dp501_fw_addr = NULL;
 				}
 			}
-			/* fallthrough */
+			__fallthrough;
 		case 0x0c:
 			ast->tx_chip_type = AST_TX_DP501;
 		}

src/drivers/ipmi/ipmi_kcs_ops.c

@@ -247,7 +247,7 @@ ipmi_write_acpi_tables(const struct device *dev, unsigned long current,
 		break;
 	default:
 		printk(BIOS_ERR, "IPMI: Unsupported register spacing for SPMI\n");
-		/* fall through */
+		__fallthrough;
 	case 1:
 		addr.bit_offset = 8;
 		break;
@@ -369,7 +369,7 @@ static int ipmi_smbios_data(struct device *dev, int *handle,
 		break;
 	default:
 		printk(BIOS_ERR, "IPMI: Unsupported register spacing for SMBIOS\n");
-		/* fall through */
+		__fallthrough;
 	case 1:
 		register_spacing = 0 << 6;
 		break;

src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c

@@ -301,13 +301,13 @@ static void sn65dsi86_bridge_valid_dp_rates(uint8_t bus, uint8_t chip, bool rate
 	default:
 		printk(BIOS_ERR, "Unexpected max rate (%#x); assuming 5.4 GHz\n",
 		       (int)dpcd_val);
-		/* fall through */
+		__fallthrough;
 	case DP_LINK_BW_5_4:
 		rate_valid[7] = 1;
-		/* fall through */
+		__fallthrough;
 	case DP_LINK_BW_2_7:
 		rate_valid[4] = 1;
-		/* fall through */
+		__fallthrough;
 	case DP_LINK_BW_1_62:
 		rate_valid[1] = 1;
 		break;