mtd: rawnand: denali: fix probe function error path

An error after nand_scan_tail() should trigger a nand_cleanup().
The helper mtd_device_register() returns an error code that should
be checked and nand_cleanup() called accordingly.

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