mtd: rawnand: s3c2410: Error out when ->nrsets < 0 or ->sets == NULL

All of the code in the probe path assumes ->sets != NULL and
->nrsets > 0. Error out if that's not the case to avoid dereferencing a
NULL pointer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>