clk: qcom: Retrieve pre_div from freq_tbl for shared RCG

There could be cases where a simultaneous clk_disable and clk_set_rate on
the same rcg could result in a wrong recalc rate. So for shared rcgs
get the pre_div value based on the current frequency from the frequency
table.

Change-Id: Ia5730e43965b2d0ba19da05fca47825514f98865
Signed-off-by: Taniya Das <tdas@codeaurora.org>