cpufreq: skip invalid entries when searching the frequency

Skip invalid entries when searching the frequency. This fixes cpufreq
at least on loongson2 MIPS board.

Fixes: da0c6dc0

 (cpufreq: Handle sorted frequency tables more efficiently)
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: 4.8+ <stable@vger.kernel.org> # 4.8+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>