kingst-la2016: comment on FPGA register at offset 0x0004 (unused)

The vendor firmware optionally keeps reading u32 values from the FPGA
register at offset 0x0004 outside of acquisition periods. To present
the input pin's current level and changes "in real time" in the GUI.
The sigrok driver does not use this information, but it's useful to
have the comment in the source code.