Fix analog output display

Fix analog output display

I've seen the following output from sigrok-cli:

CH1: 478.720 mV
CH1: -514 mV
CH1: -0 V

I added some debug, and it seems like the digits value isn't reset
to the actual value after calling sr_analog_si_prefix_friendly():

using 6 digits
value2 0.478720 digits 6
value2 -0.513536 digits 3
value2 -0.487424 digits 0

This commit fixes this by resetting the value to the actual value before.

Signed-off-by: Sven Schnelle <redacted>