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>

drivers/mmc/host/sdhci-esdhc-imx.c

@@ -695,6 +695,7 @@ static void esdhc_prepare_tuning(struct sdhci_host *host, u32 val)
 	/* FIXME: delay a bit for card to be ready for next tuning due to errors */
 	mdelay(1);
 
+	/* This is balanced by the runtime put in sdhci_tasklet_finish */
 	pm_runtime_get_sync(host->mmc->parent);
 	reg = readl(host->ioaddr + ESDHC_MIX_CTRL);
 	reg |= ESDHC_MIX_CTRL_EXE_TUNE | ESDHC_MIX_CTRL_SMPCLK_SEL |