clk: qcom: clk-alpha-pll: Fix issues with using uninitialized variables

Certain pll_vco structures might be used without initializing them
first. Initialize these structures to NULL while defining them.

Change-Id: Iadba1e659efb306bb2c2fe5caa2577e708335501
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>