mtd: block2mtd: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 402015 ("Missing break in switch")
Addresses-Coverity-ID: 402016 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/devices/block2mtd.c

@@ -329,8 +329,10 @@ static int ustrtoul(const char *cp, char **endp, unsigned int base)
 	switch (**endp) {
 	case 'G' :
 		result *= 1024;
+		/* fall through */
 	case 'M':
 		result *= 1024;
+		/* fall through */
 	case 'K':
 	case 'k':
 		result *= 1024;