PM / OPP: of_property_count_u32_elems() can return errors

of_property_count_u32_elems() will never return 0, but a -ve error value
of a positive count. And so the current !count check is wrong.

Also, a missing "opp-microvolt" property isn't a problem and so we need
to do of_find_property() separately to confirm that.

Fixes: 27465902 (PM / OPP: Add support to parse "operating-points-v2" bindings)
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>