staging: comedi: serial2002: cleanup serial2002_setup_subdevs()

Define and document the bit shifts of the serial.data read from
the device that is used to configure the subdevice channels.

Use the new defines to tidy up the configuration process.

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>