mmc: sdhci-esdhc-imx: Convert to mmc_send_tuning()

Instead of having a local function taking care of sending the tuning
command, let's use the common mmc_send_tuning() API provided by the mmc
core. In this way the request will be handled as any other request by
sdhci core.

As an effect of this change, the pm_runtime_get_sync() call at
esdhc_prepare_tuning() isn't needed any more.

This patch will also introduce another change in behavior, since before
the response pattern to the tuning command wasn't verified by
sdhci-esdhc-imx. The mmc_send_tuning() does that.

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