stm32/eth: fix h7 examples

The STM32H7 examples enable `dhcpv4` and `dns` features. Both use one
socket, meaning that we need one extra socket for our firmware tcp
socket.

examples/stm32h7/src/bin/eth.rs

@@ -92,7 +92,7 @@ async fn main(spawner: Spawner) -> ! {
     let stack = &*make_static!(Stack::new(
         device,
         config,
-        make_static!(StackResources::<2>::new()),
+        make_static!(StackResources::<3>::new()),
         seed
     ));
 

examples/stm32h7/src/bin/eth_client.rs

@@ -93,7 +93,7 @@ async fn main(spawner: Spawner) -> ! {
     let stack = &*make_static!(Stack::new(
         device,
         config,
-        make_static!(StackResources::<2>::new()),
+        make_static!(StackResources::<3>::new()),
         seed
     ));