mmc: sdhci-of-esdhc: avoid writing power control register

The eSDHC doesn't have a standard power control register, so when
writing this register in stack we should do nothing to avoid
incorrect operation.

Signed-off-by: Yangbo Lu <yangbo.lu@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>