mtd: rawnand: s3c2410: enhance the probe function error path

Prepare the migration of the lpc32xx_slc driver to use nand_scan() by
cleaning the error path in the probe function.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>