hung-chang-dso-2100: Add driver

hung-chang-dso-2100: Add driver

The Hung-Chang DSO-2100 is a parallel port PC oscilloscope sold back
in 1999 under brand names like Protek and Voltcraft.

This inital version of the driver has the following limitations:

 - Hardcoded calibration values. All parameters are set to 50%.
 - No support for auto triggering
 - No support for TV sync trigger modes
 - No support for the "scroll acquisition" mode

In scroll acquisition mode the device behaves more like a multimeter
and reports the current voltage of a probe on request. While in this
mode the sample rate is limited by the parallel port interface, it is
the only way to capture both channels at the same time (well, sort of).

Calibration would need auto triggering. The calibration values are very
temperature dependent and the device takes literally hours to reach its
final temperature. Every vdiv setting needs its own set of calibration
values. Without hardware modifications, the calibration settings wear
of in less than a second while waiting for a trigger because the
capacitors storing those values are not recharged in state 0x21.