xhci: Fix reset-device and configure-endpoint commands

We have been having problems with the USB-IF Gold Tree tests when plugging
and unplugging devices from the tree. I have seen that the reset-device
and configure-endpoint commands, which are invoked from
xhci_discover_or_reset_device() and xhci_configure_endpoint(), will sometimes
time out.

After much debugging, I determined that the commands themselves do not actually
time out, but rather their completion events do not get delivered to the right
place.

This happens when the command ring has just wrapped around, and it's enqueue
pointer is left pointing to the link TRB. xhci_discover_or_reset_device() and
xhci_configure_endpoint() use the enqueue pointer directly as their command
TRB pointer, without checking whether it's pointing to the link TRB.

When the completion event arrives, if the command TRB is pointing to the link
TRB, the check against the command ring dequeue pointer in
handle_cmd_in_cmd_wait_list() fails, so the completion inside the command does
not get signaled.

The patch below fixes the timeout problem for me.

This should be queued for the 2.6.35 and 2.6.36 stable trees.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@kernel.org