mtd: nand: fix DocBook warnings on nand_sdr_timings doc

Change the comment type (from /** to /*) to prevent DocBook from
complaining about missing description for nand_sdr_timings fields.

There is currently no need in documenting those fields because they are
fully described in the ONFI specification (which is pointed out in the
comment).

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>