blocking/spi: Don't return the same buffer back.

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - `blocking::Serial`: renamed `bwrite_all` to `write`, `bflush` to `flush.
 - Removed `prelude` to avoid method name conflicts between different flavors (blocking, nb) of the same trait. Traits must now be manually imported.
 - Removed the various `Default` marker traits.
+- Removed `&[W]` returned slice in `spi::blocking::Transfer`.
 
 ### Removed
 - Removed random number generation (`rng`) traits in favor of [rand_core](https://crates.io/crates/rand_core).

src/blocking/spi.rs

@@ -5,8 +5,10 @@ pub trait Transfer<W> {
     /// Error type
     type Error;
 
-    /// Writes `words` to the slave. Returns the `words` received from the slave
-    fn transfer<'w>(&mut self, words: &'w mut [W]) -> Result<&'w [W], Self::Error>;
+    /// Writes and reads simultaneously. The contents of `words` are
+    /// written to the slave, and the received words are stored into the same
+    /// `words` buffer, overwriting it.
+    fn transfer(&mut self, words: &mut [W]) -> Result<(), Self::Error>;
 }
 
 /// Blocking write