msm: acpuclock-8960: Don't always use HFPLL constraints

If the PLL we're switching to isn't using an HFPLL we still use
the HFPLL constraints to calculate the vdd_core and vdd_dig.
Ignore the PLL constraint if the PLL isn't an HFPLL so that
voltages can drop if possible.

Change-Id: Ieb9f356e294209adb85c642315a28db71a335390
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>