mtd: maps: vmu-flash: Fix the (mtd core) switch to ref counters

While switching to ref counters for track mtd devices use, the vmu-flash
driver was forgotten. The reason for reading the ref counter seems
debatable, but let's just fix the build for now.

Fixes: 19bfa9ebebb5 ("mtd: use refcount to prevent corruption")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Cc: [e-mail]
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/maps/vmu-flash.c

···
 	card = maple_get_drvdata(mdev);
 	for (x = 0; x < card->partitions; x++) {
 		mtd = &((card->mtd)[x]);
-		if (mtd->usecount > 0)
+		if (kref_read(&mtd->refcnt))
 			return 0;
 	}
 	return 1;