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 b92cc66c "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