pm_runtime_put_sync() gets called everytime in xhci_dbc_stop().
If dbc is not started, this makes the runtime PM counter incorrectly
becomes 0, and calls autosuspend function. Then we'll keep seeing this:
[54664.762220] xhci_hcd 0000:00:14.0: Root hub is not suspended
So only calls pm_runtime_put_sync() when dbc was started.
Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>