mtd: nand: fix nand.h kernel-doc warnings

Fix kernel-doc warnings in <linux/mtd/nand.h>:

..//include/linux/mtd/nand.h:658: warning: No description found for parameter 'tCEH_min'
..//include/linux/mtd/nand.h:877: warning: No description found for parameter 'data_interface'

Fixes: eee64b70

 ("mtd: nand: Introduce nand_data_interface")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>