dahdi_span_assignment: Run auto assignment for multiple devices in parallel by default.

This allows auto span assignment for multiple devices to happen in registration
order by default.

For example, the wcte12xp has a device in pci:0000:04:08.0 and the wcaxx in
pci:000:02:00.0. You can see that when auto assigning spans, the order matches
the driver load order:

  # /etc/init.d/dahdi stop
  Unloading DAHDI hardware modules: done
  # modprobe wcte12xp; modprobe wcaxx
  # dahdi_span_assignments auto
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:04:08.0
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:02:00.0
  # /etc/init.d/dahdi stop
  Unloading DAHDI hardware modules: done
  # modprobe wcaxx; modprobe wcte12xp
  # dahdi_span_assignments auto
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:02:00.0
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:04:08.0
  # /etc/init.d/dahdi stop
  Unloading DAHDI hardware modules: done
  # modprobe wcaxx; modprobe wcte12xp
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:02:00.0
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:04:08.0
  # /etc/init.d/dahdi stop
  Unloading DAHDI hardware modules: done
  # modprobe wcte12xp; modprobe wcaxx
  dahdi_span_assignments auto
  # dahdi_span_assignments auto
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:04:08.0
  auto-assign /sys/bus/dahdi_devices/devices/pci:0000:02:00.0

The above behavior depends on dahdi's auto_assign_spans module parameter to be
equal to 0. Otherwise, the drivers will not try to enforce any ordering.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>