xhci: Ensure a command structure points to the correct trb on the command ring

If a command on the command ring needs to be cancelled before it is handled
it can be turned to a no-op operation when the ring is stopped.
We want to store the command ring enqueue pointer in the command structure
when the command in enqueued for the cancellation case.

Some commands used to store the command ring dequeue pointers instead of enqueue
(these often worked because enqueue happends to equal dequeue quite often)

Other commands correctly used the enqueue pointer but did not check if it pointed
to a valid trb or a link trb, this caused for example stop endpoint command to timeout in
xhci_stop_device() in about 2% of suspend/resume cases.

This should also solve some weird behavior happening in command cancellation cases.

This patch is based on a patch submitted by Sarah Sharp to linux-usb, but
then forgotten:
    http://marc.info/?l=linux-usb&m=136269803207465&w=2



This patch should be backported to kernels as old as 3.7, that contain
the commit b92cc66c "xHCI: add aborting
command ring function"

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org
Change-Id: Icb4b45920331812c69e94111e462c127f03ca716
Git-commit: ec7e43e2d98173483866fe2e4e690143626b659c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jack Pham <jackp@codeaurora.org>