clk: qcom: Round parent rate in clk_rcg2_determine_rate

We currently set best_parent_rate to the ideal frequency wanted by the
RCG. This doesn't always align to the exact frequency the parent can
provide. If they differ, then clk_calc_new_rates() will return the
parent as top, even if the rounded parent frequency isn't changing.
This causes clk_propagate_rate_change() to return before being called
for its children, which means children aren't notified of the
PRE_RATE_CHANGE events.

Set best_parent_rate to the rounded rate so that we don't unnecessarily
try to change the parent.

Change-Id: Ic976db7a13dfbc63cb66b032a45060565ca90b43
Signed-off-by: Mike Tipton <mdtipton@codeaurora.org>