mmc: sdhci-acpi: enable sdhci-acpi device to suspend/resume asynchronously

This patch enables sdhci-acpi devices to suspend/resume asynchronously.
This will improve system suspend/resume speed. After enabling the
sdhci-acpi devices and all their child devices to suspend/resume
asynchronously on ASUS T100TA, the system suspend-to-idle time is
reduced from 1645ms to 1089ms, and the system resume time is reduced
from 940ms to 908ms.

Signed-off-by: Zhonghui Fu <zhonghui.fu@linux.intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci-acpi.c

@@ -418,6 +418,8 @@ static int sdhci_acpi_probe(struct platform_device *pdev)
 		pm_runtime_enable(dev);
 	}
 
+	device_enable_async_suspend(dev);
+
 	return 0;
 
 err_free: