usb/isp1760: Fix race condition memory leak

This fixes a memory leak reported by Catalin Marinas:

schedule_ptds() is called from isp1760_irq() and removes the qh from the
controlqhs queue but ep->hcpriv still points to the qh and therefore it is not
freed.

Shortly after this, the isp1760_endpoint_disable() function sets ep->hcpriv to
NULL and calls schedule_ptds() but since the corresponding qh is no longer in
the queue, it is simply forgotten and reported by kmemleak.

With this patch, the qh is always freed at endpoint_disable, instead, and the
corresponding entry removed from the queue head list.

While I was at it, I also replaced the lines in isp1760_endpoint_disable()
that removed remaining qtds from the qh with a WARN_ON check for non-empty qh,
in line with earlier comments from Alan Stern (linux-usb list, 2011-07-20).

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>