mtd: add missing breaks in a switch

On platforms supporting both: TRX and Seama calling "fixtrx" was
resulting in trying to fix Seama as well.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

package/system/mtd/src/mtd.c

@@ -969,10 +969,12 @@ int main (int argc, char **argv)
 			if (mtd_fixtrx) {
 				mtd_fixtrx(device, offset);
 			}
+			break;
 		case CMD_RESETBC:
 			if (mtd_resetbc) {
 				mtd_resetbc(device);
 			}
+			break;
 		case CMD_FIXSEAMA:
 			if (mtd_fixseama)
 				mtd_fixseama(device, 0);