gateware.iostream.IOClocker: Fix phase and state keeps flipping.

When i_stream is idle, there is currently nothing that prevents
the state machine from changing state between Falling/Rising,
toggling phase, and restarting the timer.

This results in lost power, and sometimes shortened clock pulses
after an idle period.

software/glasgow/gateware/iostream.py

@@ -285,7 +285,7 @@ def elaborate(self, platform):
  m.d.comb += self.i_stream.ready.eq(1)
  with m.Else():
  m.d.comb += phase.eq(0)
- with m.If(self.o_stream.ready):
+ with m.If(self.o_stream.ready & self.i_stream.valid):
  m.d.sync += timer.eq(self.divisor)
  m.next = "Rising"