mtd: rawnand: ams-delta: Fix various coding style issues

Most of them were reported by checkpatch:

* s/u_char/u8/
* remove unneeded blank lines
* don't print warning messages when devm_kzalloc() fails
* Use ! instead of == NULL
* Remove invalid comment

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>