dahdi_handle_device, dahdi_span_config: Check for auto_assign_spans only when ACTION is add.

When dahdi.ko is unloaded, it may be possible for the driver to be removed from
the kernel before the udev scripts are run.  When this happens, you'll see
messages like the following which are not accurate:

    'dahdi_handle_device'[24567]: Old driver (no auto_assign_spans parameter). Skip /devices/pci0000:00/0000:00:1e.0/0000:11:01.0/pci:0000:11:01.0

Now instead you will see:

    'dahdi_handle_device'[28008]: remove: /devices/pci0000:00/0000:00:1e.0/0000:11:01.0/pci:0000:11:01.0

Also, all the udev events will be logged in the system log even when they are
ignored because of legacy auto span assignment. This will help show what is
going on during the transition period to full udev configuration of spans.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Oron Peled <oron.peled@xorcom.com>