mmc: sdhci-esdhc-imx: comment runtime_pm_get_sync() in esdhc_prepare_tuning()

It is far from obvious what this is doing, and it looks like it's an
unbalanced runtime_pm_get() call.  However, the put is inside
sdhci_tasklet_finish(), so it's not unbalanced at all.  This should
be documented so people know what's going on here.  Do so.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>