usb: dwc3: host: Disable host mode runtime PM

Currently there are issues with the HS PHY not entering L2
state when in host mode and attempting to suspend the controller.
Disable xHCI's runtime PM for now while this is debugged.

Change-Id: I448fb0a002041025f6a3b4c999cb0d4d3059c6c0
Signed-off-by: Tanya Finkel <tfinkel@codeaurora.org>