usb: pd: disable VBUS/VCONN upon shutdown

Upon system shutdown make sure to disable VBUS and VCONN
to prevent the HW from being in a state where VBUS is left
on and could cause the device to power back on again.

Change-Id: I8bb181a843c323a2d3251bccf83db3685d6c82bc
Signed-off-by: Jack Pham <jackp@codeaurora.org>