hal-common: require DerefMut for peripherals, not just Deref.

Otherwise you can create multiple drivers on the same singleton like this:

```rust
let mut input = Input::new(&pin, Pull::None);
let mut output = Output::new(&pin, Level::Low, Speed::Low);
input.is_high();
output.set_high();
input.is_high();
output.set_high();
```

Thanks @pennae for reporting.

1 files changed, 2 insertions, 2 deletions

diff --git a/embassy-stm32/src/dac/mod.rs b/embassy-stm32/src/dac/mod.rs
index 1dc13949d..31a2d8863 100644
--- a/embassy-stm32/src/dac/mod.rs
+++ b/embassy-stm32/src/dac/mod.rs
@@ -264,7 +264,7 @@ impl<'d, T: Instance, Tx> DacCh1<'d, T, Tx> {
         });
 
         let tx_request = self.dma.request();
-        let dma_channel = &self.dma;
+        let dma_channel = &mut self.dma;
 
         let tx_options = crate::dma::TransferOptions {
             circular,
@@ -376,7 +376,7 @@ impl<'d, T: Instance, Tx> DacCh2<'d, T, Tx> {
         });
 
         let tx_request = self.dma.request();
-        let dma_channel = &self.dma;
+        let dma_channel = &mut self.dma;
 
         let tx_options = crate::dma::TransferOptions {
             circular,