mfd: rtsx_usb: Fix runtime PM deadlock

sd_set_power_mode() in derived module drivers/mmc/host/rtsx_usb_sdmmc.c
acquires dev_mutex and then calls pm_runtime_get_sync() to make sure the
device is awake while initializing a newly inserted card. Once it is
called during suspending state and explicitly before rtsx_usb_suspend()
acquires the same dev_mutex, both routine deadlock and further hang the
driver because pm_runtime_get_sync() waits the pending PM operations.

Fix this by using an empty suspend method. mmc_core always turns the
LED off after a request is done and thus it is ok to remove the only
rtsx_usb_turn_off_led() here.

Cc: <stable@vger.kernel.org> # v3.16+
Fixes: 730876be ("mfd: Add realtek USB card reader driver")
Signed-off-by: Roger Tseng <rogerable@realtek.com>
[Lee: Removed newly unused variable]
Signed-off-by: Lee Jones <lee.jones@linaro.org>