mtd: rawnand: fix kernel-doc warnings

Fix kernel-doc warnings in drivers/mtd/nand/raw:

../drivers/mtd/nand/raw/nand_base.c:420: warning: Function parameter or member 'chip' not described in 'nand_fill_oob'
../drivers/mtd/nand/raw/nand_bbt.c:173: warning: Function parameter or member 'this' not described in 'read_bbt'
../drivers/mtd/nand/raw/nand_bbt.c:173: warning: Excess function parameter 'chip' description in 'read_bbt'

Fixes: 0813621b

 ("mtd: rawnand: Stop passing mtd_info objects to internal functions")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Boris Brezillon <boris.brezillon@bootlin.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: linux-mtd@lists.infradead.org
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>