api: adc: exists needs to handle #channels

This is a bug in the kernel which will be fixed in future releases but
for now we enable the ADC example to work.

apis/adc/src/lib.rs

@@ -10,7 +10,12 @@ pub struct Adc<S: Syscalls>(S);
 impl<S: Syscalls> Adc<S> {
  /// Returns Ok() if the driver was present.This does not necessarily mean
  /// that the driver is working.
- pub fn exists() -> Result<(), ErrorCode> {
+ //
+ // Note! The "exists" command should return `Result<(), ErrorCode>`, but the
+ // current ADC driver in the kernel returns the number of ADC channels
+ // instead of just success. This will be fixed in a future release of Tock,
+ // but for now we workaround this issue.
+ pub fn exists() -> Result<u32, ErrorCode> {
  S::command(DRIVER_NUM, EXISTS, 0, 0).to_result()
  }