eth: implement DeviceCapabilities.max_burst_size

this is a hint that /could/ boost TCP performance.

libboard_zynq/src/eth/mod.rs

@@ -370,6 +370,7 @@ impl<'r, 'a> smoltcp::phy::Device<'a> for &mut Eth<'r, rx::DescList, tx::DescLis
 
         let mut caps = DeviceCapabilities::default();
         caps.max_transmission_unit = MTU;
+        caps.max_burst_size = Some(self.rx.len().min(self.tx.len()));
         caps.checksum = checksum_caps;
 
         caps

libboard_zynq/src/eth/rx.rs

@@ -93,6 +93,10 @@ impl DescList {
         }
     }
 
+    pub fn len(&self) -> usize {
+        self.list.len().min(self.buffers.len())
+    }
+
     pub fn list_addr(&self) -> u32 {
         &self.list[0] as *const _ as u32
     }

libboard_zynq/src/eth/tx.rs

@@ -85,6 +85,10 @@ impl DescList {
         }
     }
 
+    pub fn len(&self) -> usize {
+        self.list.len().min(self.buffers.len())
+    }
+
     pub fn list_addr(&self) -> u32 {
         &self.list[0] as *const _ as u32
     }