The composite clock's .determine_rate implementation can call the
underyling .determine_rate callback corresponding to rate_hw or the
underlying .determine_rate callback corresponding to mux_hw. In both
cases we pass in rate_hw, which is wrong. Fixed by passing mux_hw into
the correct callback.
Reported-by: Lemon Dai <dailemon.gl@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>