mtd: rawnand: ams-delta: Write protect device during probe

Initialise NWP GPIO pin as asserted to protect the device from hazard
during setup of other GPIO pins.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-2-jmkrzyszt@gmail.com