mtd: Fixed breaking list in __mtd_del_partition.

Not the child partition should be removed from the partition list
but the partition itself. Otherwise the partition list gets broken
and any subsequent remove operations leads to a kernel panic.

Fixes: 46b5889c ("mtd: implement proper partition handling")
Signed-off-by: Andreas Oetken <andreas.oetken@siemens-energy.com>
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211102172604.2921065-1-andreas.oetken@siemens-energy.com