staging: comedi: refactor comedi_device_attach() a bit

Split the post-config part of comedi_device_attach() into new function
comedi_device_postconfig() and rearrange the rest of the function a bit.

The new comedi_device_postconfig() function will be called by some new
bus-type-specific auto-attach functions.

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