tb: Use correct width for Xs

In several cases, not the number of Xs did not match the number of bits.
Correct this.

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

tb/mdio_regs.py

@@ -51,7 +51,7 @@ async def wb_xfer(signals, addr, data=None):
     assert signals['ack'].value or signals['err'].value
     signals['stb'].value = 0
     signals['we'].value = LogicArray('X')
-    signals['addr'].value = LogicArray('X' * 4)
+    signals['addr'].value = LogicArray('X' * len(signals['addr']))
     signals['data_write'].value = LogicArray('X' * 16)
     if data is None and signals['ack'].value:
         return signals['data_read'].value

tb/nrzi_decode.py

@@ -21,7 +21,7 @@ async def nrzi_decode(bits):
 @timeout(10, 'us')
 async def test_rx(decoder, valids):
     decoder.nrzi_valid.value = 0
-    decoder.nrzi.value = LogicArray('X', 'X')
+    decoder.nrzi.value = LogicArray('XX')
     decoder.rst.value = 1
     await Timer(1)
     await cocotb.start(Clock(decoder.clk, 8, units='ns').start())