mmc: mxs-mmc: No need to do NULL check on 'iores'

devm_ioremap_resource() already checks if 'iores' is NULL or not, so we can
skip this manual check.

While at it, move platform_get_resource() closer to devm_ioremap_resource() for
better readability.

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