mtd: atmel_nand: fix error return code in atmel_nand_probe()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>