If the lower bits of m_ptr is the same as s_ptr when we get s_axis_last
(that is, we are full), then we will immediately end (since m_ptr will
equal last_ptr). Fix this by including all of s_ptr in last_ptr.
Fixes: 52325f2 ("Add AXI stream replay buffer")
Signed-off-by: Sean Anderson <seanga2@gmail.com>