usb: phy: msm: Disable runtime PM for root hub upon usb disconnect

There is a possibility of runtime idle thread for root hub
being scheduled out inside autosuspend_check() during root hub
device disconnect. Device interface pointer is set to NULL by
usb_disconnect as part of device disconnect. This results into
NULL pointer dereference when runtime idle thread gets
a chance to run and access it later in side autosuspend_check().
Hence disable runtime PM for root hub and finish any pending
runtime PM request synchronously before disconnecting root hub device.

Change-Id: I24e290e8061a676ea04c5028d531da865be2d937
Signed-off-by: Sai Krishna Juturi <jsaikrishna@codeaurora.org>