mtd: rawnand: sunxi: Fix kernel doc headers

Fix the struct description and use standard kernel-doc header format
(even if the file is not parsed by the doc generator).

We also replace tabs by a single space.

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