clk: qcom: clk-spmi-pmic-div: avoid potential divide-by-0

Add a check of the cxo_hz value to ensure that it is greater than
0.  That way, there is no chance of division by zero when using
cxo_hz to calculate cxo_period_ns.

Change-Id: I5498b7a08e4cb3be45469041a973d045f2ec2bdb
Signed-off-by: David Collins <collinsd@codeaurora.org>