staging: comedi: usbdux: tidy up usbdux_attach_common()

Rename the local variable used for the device private data.

Move the setting of the device private data 'comedidev' to the
(*auto_attach) where the other back pointers are set.

Tidy up the subdevice init by removing the unnecessary comments
and adding some whitespace.

Remove the unnecessary dev_info() after a sucessful attach.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>