mtd: core: don't remove debugfs directory if device is in use

Previously, if del_mtd_device() failed with -EBUSY due to a non-zero
usecount, a subsequent call to attempt the deletion again would try to
remove a debugfs directory that had already been removed and panic.
With this change the second call can instead proceed safely.

Fixes: e8e3edb9 ("mtd: create per-device and module-scope debugfs entries")
Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211014203953.5424-1-zev@bewilderbeest.net