usb: pd: Prevent sleep when state machine in progress

With PM autosleep enabled, the system may aggressively enter
and re-enter suspend in between calls to queue_work() and
when the state machine function gets executed, which may
at best result in numerous suspend/resume cycles and at
worse starve the PD state machine from even executing.

Fix this by calling pm_stay_awake() whenever the work is
queued and relinquish with pm_relax() when no further work
is to be done.

Change-Id: I71d8ea0c69a7b174c2280f92d1f5342e5e50d9b0
Signed-off-by: Jack Pham <jackp@codeaurora.org>