mtd: rawnand: nandsim: Consistent use of 'ns' instead of 'dev'

The nandsim object is called 'ns' almost everywhere, keep it that way
everywhere for consistency.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-2-miquel.raynal@bootlin.com