mtd: rawnand: ams-delta: cleanup ams_delta_init() error path

Remove unused labels, rename out_mtd into err_unmap to make it clearer
and return 0 instead of using a goto out at the end of the registration
procedure.

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>