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.

---
Changes in v2:
- Update commit message

 drivers/mmc/host/sdhci-acpi.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index f6047fc..3d27f2d 100644
@@ -388,6 +388,8 @@ static int sdhci_acpi_probe(struct platform_device *pdev)
 		pm_runtime_enable(dev);
 	}
 
+	device_enable_async_suspend(dev);
+
 	return 0;
 
 err_free:

-- 1.7.1