mmc: mxs-mmc: Propagate the real error

If platform_get_irq() fails, it is better to propagate the real error value
instead of a 'fake' one.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>