tpm: Only call pm_runtime_get_sync if device has a parent

Only call pm_runtime_get_sync if the device has a parent. This
change fixes a crash in the tpm_vtpm_proxy driver since that
driver does not have a parent device.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>