MIPS: Fix gen_nand probe structures contents

    
This platform didn't properly fill nr_chips in gen_nand
registration and therefore depended on gen_nand bug fixed by commit
81cbb0b1 ("mtd: gen_nand: fix support for
multiple chips")
    
Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>