mtd: rawnand: lpc32xx_slc: fix the probe function error path

An error after nand_scan_tail() should trigger a nand_cleanup() and not
a nand_release(). The latter doing an mtd_device_unregister() which is
not needed if mtd_device_register() failed.

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