mtd: nand_flash_detect_onfi propagate busw info

there is a bug in nand_flash_detect_onfi, busw need to be passed
by pointer to return it.

Signed-off-by: Matthieu CASTET <matthieu.castet@parrot.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>