clk: qcom: use divider_ro_round_rate helper

There is now an helper function to round the rate when the
divider is read-only. Let's use it

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>