staging: comedi: make attach handler optional

Some low-level Comedi drivers no longer support manual configuration of
devices with the COMEDI_DEVCONFIG ioctl (used by the comedi_config
program).  For those drivers, the 'attach_pci' or 'attach_usb' handler
will be set in the struct comedi_driver to configure devices
automatically (via comedi_pci_auto_config() or
comedi_usb_auto_config()).  Their 'attach' handlers are redundant but
the the comedi core module currently requires it to be set.

Make the 'attach' handler optional and issue a warning if something
wants to call it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>