CLK: SPEAr: Correct index scanning done for clock synths

The patch corrects the case when the rate table is being scanned for a
given frequency, and the search frequency is beyond the maximum
frequency indexed in the table.

By default, the system should be set at max frequency present in the
rate table. This patch correctly returns the corresponding index value.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>