usb, xhci: Clear warm reset change event during init

I noticed on my Panther Point system that I wasn't getting hotplug events
for my usb3.0 disk on a usb3 port.  I tracked it down to the fact that the
system had the warm reset change bit still set.  This seemed to block future
events from being received, including a hotplug event.

Clearing this bit during initialization allowed the hotplug event to be
received and the disk to be recognized correctly.

This patch should be backported to kernels as old as 2.6.39.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org