cmd: mtdparts: add fallthrough in switch statement

Switch blocks for deriving size naturally use fallthrough between
'case' statements. Make it explicit.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
diff --git a/cmd/mtdparts.c b/cmd/mtdparts.c
index 0da3afd..756fc60 100644
--- a/cmd/mtdparts.c
+++ b/cmd/mtdparts.c
@@ -177,13 +177,16 @@
 		case 'G':
 		case 'g':
 			ret <<= 10;
+			/* Fallthrough */
 		case 'M':
 		case 'm':
 			ret <<= 10;
+			/* Fallthrough */
 		case 'K':
 		case 'k':
 			ret <<= 10;
 			(*retptr)++;
+			/* Fallthrough */
 		default:
 			break;
 	}