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;
 			}