mmc: sdhci-esdhc-imx: fix runtime pm unbalance issue

Since we're using common esdhc_send_command for tuning commands and
the core code will call pm_runtime_put after command is finished.
So we add a pm_runtime_get_sync here to get the balance.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>