mtd: nand: gpmi: Check for scan_bbt() error

In case of scan_bbt() failure, we should better propagate it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>