mtd: rawnand: trivial spelling

The word 'descriptor' is misspelled throughout the tree.

Fix it up accordingly:
    decriptors -> descriptors

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200609124610.3445662-5-kieran.bingham+renesas@ideasonboard.com