xhci: Fix oops happening after address device timeout

xhci: Fix oops happening after address device timeout

When a command times out, the command ring is first aborted,
and then stopped. If the command ring is empty when it is stopped
the stop event will point to next command which is not yet set.
xHCI tries to handle this next event often causing an oops.

Don't handle command completion events on stopped cmd ring if ring is
empty.

This patch should be backported to kernels as old as 3.7, that contain
the commit 4e07e76e317e1b7ea18252d92337fc361d574d30 "xHCI: add aborting
command ring function"

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Reported-by: Giovanni <giovanni.nervi@yahoo.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org