mtd: rawnand: nandsim: Use octal permissions

Symbolic permissions 'S_IRUSR' are not preferred. Checkpatch.pl
advises to use octal permissions '0400'.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-3-miquel.raynal@bootlin.com