adc_disable_channel_differential_input does not correctly set config bit

https://github.com/arduino/ArduinoCore-sam/blob/c893c62ec9953ed36a256b5243272fda2e473c14/system/libsam/source/adc.c#L632-L635

By ORing with the previous value, the resulting of ANDing with the mask is undone, so the function results in no action being taken (once adc_enable_channel_differential_input is run, adc_disable_channel_differential_input doesn't work).
trivially with one bit, the truth table for (A and B) or A is:

A|B|(A and B) or A
|---|---|---|
T|T|T
T|F|T
F|T|F
F|F|F

I believe the desired result could be obtained by ORing with the set value (basically running adc_enable_channel_differential_input), then XORing with the same value, but I'm not a good enough C programmer to know if this is the optimal way to do this.

Start in system/libsam/source/adc.c at lines 632-635 and inspect adc_disable_channel_differential_input alongside adc_enable_channel_differential_input. Verify the register mask behavior using the issue's truth table, then confirm that disabling a channel after enabling differential input clears the differential-input configuration.

Written by the indexing model from the issue text.