selftests/intel_pstate: Improve test, minor fixes

A few changes improve the overall usability of the test:
* fix a hard-coded maximum frequency (3300),
* don't adjust the CPU frequency if only evaluating results,
* fix a comparison for multiple frequencies.

A symptom of that last issue looked like this:
  ./run.sh: line 107: [: too many arguments
  ./run.sh: line 110: 3099
  3099
  3100-3100: syntax error in expression (error token is \"3099
  3100-3100\")

Because a check will count how many differente frequencies
there are among the CPUs of the system, and after they are
tallied another read is performed, which might produce
different results.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>