mmc: tmio: Avoid boilerplate code in ->runtime_suspend()

Rather than checking the 'runtime_synced' flag each time the
->runtime_suspend() callback is invoked, let's convert into using
dev_pm_domain_start() during ->probe() and drop the corresponding
boilerplate code.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>