clk: rockchip: fix mmc get phase

If the mmc clock has no rate, it can be assumed to be constant.
In such case, there is no measurable phase shift. Just return 0
in this case instead of returning an error.

Fixes: 27608786

 ("clk: Bail out when calculating phase fails during clk registration")
Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20200303192956.64410-1-jbrunet@baylibre.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>