mtd: fsl-quadspi: Remove unnecessary 'map_failed' label

There is no need to keep the 'map_failed' label. We can simply return the error
code directly and let the code shorter and cleaner.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Han Xu <han.xu@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>