The 'open' method in struct comedi_device_struct now returns 0 on success

The 'open' method in struct comedi_device_struct now returns 0 on success
and a negative errno on failure.  The 'open' method must clean up after
itself on failure as the 'close' method won't be called.  A failure causes
the 'open' file operation for the comedi device to fail.

The dt9812 and serial2002 drivers now make use of the ability to return
an error.