Write ADC configuration instead of modifying it

This change guards against some unexpected leftover configuration from
earlier interferring with the desired operation of the ADC.

Author: hannobraun

src/adc.rs

@@ -264,7 +264,7 @@ impl<State> Adc<State> {
         where
             Chan: Channel<Adc<Ready>, ID=u8>,
     {
-        self.rb.cfgr1.modify(|_, w| {
+        self.rb.cfgr1.write(|w| {
             w
                 .res().bits(self.precision as u8)
                 .cont().bit(cont)