tb: pcs_rx: Allow err to be optional

The downstream TX side of the MII is almost the same as the upstream RX
side of the MII. However, MACs may never generate TX_ER, so I will be
leaving it out. TO allow testing this, make the err signal optional.

Signed-off-by: Sean Anderson <seanga2@gmail.com>

tb/pcs_rx.py

@@ -40,7 +40,7 @@ async def mii_recv_packet(pcs, signals=None):
 
     while signals['valid'].value:
         if signals['ce'].value:
-            if signals['err'].value:
+            if 'err' in signals and signals['err'].value:
                 yield None
             else:
                 yield signals['data'].value