clk: qcom: alpha: Update the setrate/enable for slew PLL

Currently the slew PLL would get calibrated each time the PLL enable is
requested which could result in L/Alpha values set by the PLL set rate.
Avoid the above condition by calibrating the PLL in the set rate path.

Change-Id: Ib83129a7f8ad5c0c8535a7f10c2e60573c06dc1f
Signed-off-by: Taniya Das <tdas@codeaurora.org>