analog: add support for negative number of digits

analog: add support for negative number of digits

When a meter display 105.2 kΩ, libsigrok will return 105200 Ω
but it is really valuable to know that the last 2 digits are not
significant, so encoding.digits should be set to -2.
This would allow a sigrok client to display 105200 as 105.2 k
instead of 105.200 k.