Remove default impl for Device::limits().

We should not assume any default MTU.

src/phy/loopback.rs

@@ -12,7 +12,7 @@ use std::collections::VecDeque;
 use collections::{Vec, VecDeque};
 
 use Error;
-use super::Device;
+use super::{Device, DeviceLimits};
 
 /// A loopback interface.
 #[derive(Debug)]
@@ -32,6 +32,13 @@ impl Device for Loopback {
     type RxBuffer = Vec<u8>;
     type TxBuffer = TxBuffer;
 
+    fn limits(&self) -> DeviceLimits {
+        DeviceLimits {
+            max_transmission_unit: 65535,
+            ..DeviceLimits::default()
+        }
+    }
+
     fn receive(&mut self) -> Result<Self::RxBuffer, Error> {
         match self.0.borrow_mut().pop_front() {
             Some(packet) => Ok(packet),

src/phy/mod.rs

@@ -162,9 +162,7 @@ pub trait Device {
     type TxBuffer: AsRef<[u8]> + AsMut<[u8]>;
 
     /// Get a description of device limitations.
-    fn limits(&self) -> DeviceLimits {
-        DeviceLimits::default()
-    }
+    fn limits(&self) -> DeviceLimits;
 
     /// Receive a frame.
     ///