Fix GCC7 warnings about switch-case fallthroughs

GCC7 with -Wextra warns about switch-case blocks which fallthrough with
"this statement may fall through [-Werror=implicit-fallthrough=]". This
can be fixed by adding "special" comments: "/* fallthrough */".

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Iba0be791dbdd86984489b2d9a0592bb59828da1e
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4174
Tested-by: jenkins

src/flash/mflash.c

@@ -259,11 +259,11 @@ static int mg_dsk_wait(mg_io_type_wait wait_local, uint32_t time_var)
 				case mg_io_wait_rdy:
 					if (status & mg_io_rbit_status_ready)
 						return ERROR_OK;
-
+					/* fallthrough */
 				case mg_io_wait_drq:
 					if (status & mg_io_rbit_status_data_req)
 						return ERROR_OK;
-
+					/* fallthrough */
 				default:
 					break;
 			}

src/flash/nand/mx3.c

@@ -281,6 +281,7 @@ static int imx31_command(struct nand_device *nand, uint8_t command)
 			 * offset == one half of page size
 			 */
 			in_sram_address = MX3_NF_MAIN_BUFFER0 + (nand->page_size >> 1);
+			break;
 		default:
 			in_sram_address = MX3_NF_MAIN_BUFFER0;
 	}

src/flash/nor/kinetis.c

@@ -2126,6 +2126,7 @@ static int kinetis_probe_chip(struct kinetis_chip *k_chip)
 			case KINETIS_SDID_FAMILYID_K6X | KINETIS_SDID_SUBFAMID_KX1:	/* errata 7534 - should be K63 */
 			case KINETIS_SDID_FAMILYID_K6X | KINETIS_SDID_SUBFAMID_KX2:	/* errata 7534 - should be K64 */
 				subfamid += 2; /* errata 7534 fix */
+				/* fallthrough */
 			case KINETIS_SDID_FAMILYID_K6X | KINETIS_SDID_SUBFAMID_KX3:
 				/* K63FN1M0 */
 			case KINETIS_SDID_FAMILYID_K6X | KINETIS_SDID_SUBFAMID_KX4: