mtd: rawnand: ams-delta: Check mtd_device_register() return code

mtd_device_register() can fail, and when it does we should propagate
the error and cleanup what has been done before.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>