mtd: core: Fix refcounting for unpartitioned MTDs

mtd: core: Fix refcounting for unpartitioned MTDs

commit 1ca71415f075353974524e96ed175306d8a937a8 upstream.

Apply changes to usecount also to the master partition.
Otherwise we have no refcounting at all if an MTD has no partitions.

Cc: stable@vger.kernel.org
Fixes: 46b5889cc2c5 ("mtd: implement proper partition handling")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201206202220.27290-1-richard@nod.at
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>