mtd: fsl-quadspi: Propagate the error from of_modalias_node()

The 'map_failed' label will return 'ret', so we need to assign the error
code to 'ret', otherwise the probe function will return success.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>