mtd: mxc_nand: Propagate the error if platform_get_irq() fails

Check the return value from platform_get_irq() and propagate it in the case of
error.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>