Staging: comedi: drivers: comedi_test: Add auto-configuration capability

Currently this module needs to be manually configured by COMEDI
userspace tool before the test waveform can be read by a COMEDI
compatible application.

This patch adds auto-configuration capability and makes it the default
loading option. This is achieved by creating a device during init
to stand in for a real hardware device. This allows comedi_auto_config()
to perform auto-configuration. With this patch, the test waveform can
be read by a COMEDI compatible application without needing manual
configuration.

Previous behaviour is still selectable via module loading parameter.
Module loading without passing any parameter will default to
auto-configuration with the same default waveform amplitude and
period values. For auto-configuration, different amplitude and
period values can be set via module loading parameters.

Tested on Xubuntu 16.04 using Xoscope ver: 2.0 which is available
in the Ubuntu repository. Xoscope is a COMEDI compatible digital
oscilloscope application. For manual configuration, only module
loading/unloading is tested.

Here are the truncated dmesg output.
[sudo modprobe comedi_test]

comedi_test: 1000000 microvolt, 100000 microsecond waveform attached
driver 'comedi_test' has successfully auto-configured 'comedi_test'.

[sudo modprobe comedi_test amplitude=2500000 period=150000]

comedi_test: 2500000 microvolt, 150000 microsecond waveform attached
driver 'comedi_test' has successfully auto-configured 'comedi_test'.

[sudo modprobe comedi_test noauto=1]

comedi_test: module is from the staging directory, the quality is unknown,
you have been warned.

For those without an actual hardware, the comedi_test module
is as close as one can get to test the COMEDI system.
Having both auto and manual configuration capability will broaden
the test function of this module.
Hopefully this will make it easier for people to check out the
COMEDI system and contribute to its development.

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>