Tweak PR267

PR267 is here:
  rust-embedded#267

I forked rust-embedded/discovery, then cloned my fork from master
and added upstream to my clone so the remotes for it are:
  wink@3900x:~/prgs/rust/tutorial/embedded-discovery/src/11-usart (fix-for-11-usart-using-pr267-and-tweaks)
  $ git remote -v
  origin  git@github.com:winksaville/rust-embedded-discovery.git (fetch)
  origin  git@github.com:winksaville/rust-embedded-discovery.git (push)
  upstream        git@github.com:rust-embedded/discovery.git (fetch)
  upstream        git@github.com:rust-embedded/discovery.git (push)

I then fetched PR267 with:
  git fetch upstream pull/267/head:fix-for-11-usart-using-pr267-and-tweaks
  git checkout fix-for-11-usart-using-pr267-and-tweaks

And then modified src/main.rs as follows:
  wink@3900x:~/prgs/rust/tutorial/embedded-discovery/src/11-usart (fix-for-11-usart-using-pr267-and-tweaks)
  $ git diff src/main.rs
  diff --git a/src/11-usart/src/main.rs b/src/11-usart/src/main.rs
  index 16c26e8..2357754 100644
  --- a/src/11-usart/src/main.rs
  +++ b/src/11-usart/src/main.rs
  @@ -1,4 +1,4 @@
  -#![deny(unsafe_code)]
  +//#![deny(unsafe_code)]
   #![no_main]
   #![no_std]

  @@ -6,11 +6,12 @@
   use aux11::{entry, iprint, iprintln};

   #[entry]
  +#[allow(unused_variables)]
   fn main() -> ! {
       let (usart1, mono_timer, itm) = aux11::init();

       // Send a single character
  -    usart1.tdr.write(|w| w.tdr().bits(u16::from(b'X')));
  +    unsafe { usart1.tdr.write(|w| w.tdr().bits(u16::from(b'X'))); }

       loop {}
   }

src/11-usart/src/main.rs

@@ -1,16 +1,17 @@
-#![deny(unsafe_code)]
+//#![deny(unsafe_code)]
 #![no_main]
 #![no_std]
 
 #[allow(unused_imports)]
 use aux11::{entry, iprint, iprintln};
 
 #[entry]
+#[allow(unused_variables)]
 fn main() -> ! {
     let (usart1, mono_timer, itm) = aux11::init();
 
     // Send a single character
-    usart1.tdr.write(|w| w.tdr().bits(u16::from(b'X')));
+    unsafe { usart1.tdr.write(|w| w.tdr().bits(u16::from(b'X'))); }
 
     loop {}
 }