Implement ADC self-calibration (#233)

* Implement ADC self-calibration

Implement the ACD self-calibration procedure, which is defined in the
RM377 reference manual as follows:

The ADC has a calibration feature. During the procedure, the ADC
calculates a calibration factor which is internally applied to the ADC
until the next ADC power-off. The application must not use the ADC
during calibration and must wait until it is complete.  Calibration
should be performed before starting A/D convers

* Wait for hardware to clear ADCAL after calibration

The reference manual says that software should wait until ADCAL = 0.
Do that instead of being paranoid and eventually timing out if this does
not happen.

---------

Co-authored-by: Javier Cardona <javier@cardonabits.com>

src/adc.rs

@@ -123,6 +123,23 @@ impl Adc<Ready> {
         self.precision = precision;
     }
 
+    /// Trigger internal ADC calibration
+    ///
+    /// This process is documented in Reference Manual RM377, section 13.3.3
+    /// Calibration (ADCAL)
+    pub fn calibrate(&mut self) -> Result<(), Error> {
+        if self._state != Ready
+            || self.rb.cr.read().aden().bit_is_set()
+            || self.rb.cfgr1.read().dmaen().bit_is_set()
+        {
+            return Err(Error::InvalidAdcState);
+        }
+
+        self.rb.cr.modify(|_, w| w.adcal().set_bit());
+        while self.rb.cr.read().adcal().bit_is_set() {}
+        Ok(())
+    }
+
     /// Starts a continuous conversion process
     ///
     /// The `channel` argument specifies which channel should be converted.
@@ -321,6 +338,7 @@ where
 }
 
 /// Indicates that the ADC peripheral is ready
+#[derive(PartialEq)]
 pub struct Ready;
 
 /// Indicates that the ADC peripheral is performing conversions
@@ -615,6 +633,9 @@ pub enum Error {
     /// just keeps writing more values. It does mean that some values in the
     /// buffer were overwritten though.
     BufferOverrun,
+
+    /// Invalid ADC state for requested operation
+    InvalidAdcState,
 }
 
 macro_rules! int_adc {