usb: host: xhci-plat: Add missing pm_runtime_mark_last_busy()

This change makes sure to use pm_runtime_mark_last_busy() at
the end of the probe() function in order to ensure that
autosuspend happens after sufficient delay. Otherwise the
controller could end up suspending too soon before a child
device has had a chance to enumerate.

In addition, fix the runtime_idle() callback to also call
mark_last_busy() as well as pm_runtime_autosuspend() directly.
This ensures that even when the child devices have suspended
that there will be an autosuspend delay enforced before allowing
the controller parent to suspend.

Change-Id: Iba58c89cf6080d828a1e28a5c152dad2df362b21
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>