clk: mmp: try to use closer one when do round rate

The orignal code will use the bigger rate between
"previous rate" and "current rate" when caculate the
rate.
In fact, hardware cares about the closest one.
So choose the closer rate between "previous rate" and
"current rate".

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>