fixed default impl

Author: ryankurte

src/blocking/spi.rs

@@ -130,11 +130,11 @@ pub mod transactional {
 
     /// Default implementation of `blocking::spi::Transactional<W>` for implementers of
     /// `spi::Write<W>` and `spi::Transfer<W>`
-    pub trait Default<W, E> {}
+    pub trait Default<W>: Write<W> + Transfer<W> {}
 
     impl<W: 'static, E, S> super::Transactional<W> for S
     where
-        S: self::Default<W, E> + Write<W, Error = E> + Transfer<W, Error = E>,
+        S: self::Default<W> + Write<W, Error=E> + Transfer<W, Error=E> ,
         W: Copy + Clone,
     {
         type Error = E;