xhci: hsic: Disable runtime PM for root hub upon driver unbind

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
xhci driver unbind 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.

CRs-Fixed: 685770
Change-Id: I4a10c3d5cb2656b8f4a1a511da418737284494f3
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>