clk: rockchip: Don't yell about bad mmc phases when getting

At boot time, my rk3288-veyron devices yell with 8 lines that look
like this:
  [    0.000000] rockchip_mmc_get_phase: invalid clk rate

This is because the clock framework at clk_register() time tries to
get the phase but we don't have a parent yet.

While the errors appear to be harmless they are still ugly and, in
general, we don't want yells like this in the log unless they are
important.

There's no real reason to be yelling here.  We can still return
-EINVAL to indicate that the phase makes no sense without a parent.
If someone really tries to do tuning and the clock is reported as 0
then we'll see the yells in rockchip_mmc_set_phase().

Fixes: 4bf59902 ("clk: rockchip: Prevent calculating mmc phase if clock rate is zero")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>