clk: do not vote for vdd_class level with 0 Hz rate_max

Inside of the clk_find_vdd_level() function when searching for
the vdd_class level associated with a given clock frequency,
ignore levels which have rate_max[] of 0 Hz.  This ensures that
the lowest non-OFF level is requested for uninitialized clock
rates of 0 Hz.

Change-Id: I83ed8564087728bd2854087f2e45f83319c6eb92
Signed-off-by: David Collins <collinsd@codeaurora.org>