Update wiring_analog.cpp

[alessandromrc]

Fixes a warning about comparing an integer to an unsigned integer

[facchinm]

Merged, thanks for your contribution!

[alessandromrc]

@facchinm I wanted to ask you if there's a possibility to avoid the Nicla Voice from always failing the test as it can mislead people when they see a board failing to compile

[facchinm]

Sure, I'm trying to understand which revision of arduino-audio-tools library introduced the regression (and pin to a previous version)

[facchinm]

Should be fixed by #668