Signals modified by cocotb tasks may not be visible to other tasks on
the same clock cycle. This was causing issues for recv_packet, because
it might not see the same values for ready/valid driven by ClockEnable
that the DUT sees. This was worked around by sampling on the RisingEdge.
However, this can cause recv_packet to miss data. Fix this by using
RisingEdge for ClockEnable, so everything can be sampled on the
FallingEdge.
Fixes: 52325f2 ("Add AXI stream replay buffer")
Signed-off-by: Sean Anderson <seanga2@gmail.com>