Do not set flash wait states to 1 by default

Back when I implemented this, I was under the assumption that 1 wait
state was a safe default value that should work for all configurations,
although not always be optimal. This turned out to be wrong.

I've seen a weird case, where the USB peripheral could get into a state
where it would no longer send data. This only happened with the number
of wait states set to 1. I don't know how other voltage range and core
frequency configurations might affect this.

I think the best option to now is to leave this configuration up to the
user, which is why I've added a method to the Flash API to configure it.

Author: hannobraun

src/flash.rs

@@ -6,7 +6,10 @@
 use cortex_m::interrupt;
 
 use crate::{
-    pac,
+    pac::{
+        self,
+        flash::acr::LATENCY_A,
+    },
     rcc::Rcc,
 };
 
@@ -75,11 +78,6 @@ impl FLASH {
         // Enable the peripheral interface
         rcc.rb.ahbenr.modify(|_, w| w.mifen().set_bit());
 
-        // Set the number of wait states to 1. This is never wrong, but will
-        // lead to suboptimal performance at lower frequencies. See STM32L0x2
-        // reference manual, section 3.3.3.
-        flash.acr.modify(|_, w| w.latency().set_bit());
-
         Self {
             flash,
             flash_end,
@@ -88,6 +86,18 @@ impl FLASH {
         }
     }
 
+    /// Set wait states
+    ///
+    /// By default, the number of wait states is zero. This is not suitable for
+    /// all configurations. Depending on the processor's voltage range and
+    /// frequency, it might be necessary to set the number of wait states to 1.
+    ///
+    /// This is explained, for example, in the STM32L0x2 Reference Manual,
+    /// section 3.3.3.
+    pub fn set_wait_states(&mut self, wait_states: LATENCY_A) {
+        self.flash.acr.modify(|_, w| w.latency().variant(wait_states));
+    }
+
     /// Erases a page of flash memory
     ///
     /// Attention: You must make sure that your program is not executed from the