mtd: set master partition panic write flag

Check and set master panic write flag so that low level drivers
can use it to take required action to ensure oops data gets written
to assigned mtdoops device partition.

Fixes: 9f897bfd ("mtd: Add flag to indicate panic_write")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200615155134.32007-1-kdasu.kdev@gmail.com