Modify read-memory to not print bitrate if outputting to stdout

- If writing to stdout, don't print the bitrate. This way, the output is
  only the data. Bitrate is still printed if data is stored directly to
  a file.

Example command to read an image from memory to stdout:
./opentitantool --conf nexus.json --interface nexus spi read-memory --length 76800 --start 1509949440 -

Change-Id: I3253d5301547a67880ca49dd3ffa107b4ff50e5d
diff --git a/sw/host/opentitantool/src/command/spi.rs b/sw/host/opentitantool/src/command/spi.rs
index 5923f00..da9549e 100644
--- a/sw/host/opentitantool/src/command/spi.rs
+++ b/sw/host/opentitantool/src/command/spi.rs
@@ -407,15 +407,15 @@
 
         if self.filename.to_str() == Some("-") {
             self.write_file(io::stdout(), &buffer)?;
+            Ok(None)
         } else {
             let file = File::create(&self.filename)?;
             self.write_file(file, &buffer)?;
+            Ok(Some(Box::new(SpiReadMemoryResponse {
+                length: buffer.len(),
+                bytes_per_second: buffer.len() as f64 / duration,
+            })))
         }
-
-        Ok(Some(Box::new(SpiReadMemoryResponse {
-            length: buffer.len(),
-            bytes_per_second: buffer.len() as f64 / duration,
-        })))
     }
 }