Refactor and bugfix the delay adding code for analog capture

Refactor and bugfix the delay adding code for analog capture

There're two essential modifications in this change:

1. Fix timing for capture frequencies less than 200 kHz;
2. Sample ADC output after CLK is set low, as per the TLC5510
datasheet.

The second point is also important as the capture state sets CLK low,
so with the previous code there might have been a very short glitch on
this line during capture. Not confirmed by measurements, but it's
cleaner this way.

Signed-off-by: Paul Fertser <redacted>