mtd: gen_nand: fix support for multiple chips

This patch corrects a problem where gen_nand driver assumed there can be only
one chip and ignored the pdata->chip.nr_chips value.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>