pm: cpupower: bench: print config file path when open cpufreq-bench.conf fails

Print out the config file path when fopen failed. It will be easy
for users to know where to create the file.

Since we are here, use strerror to drop the usage of perror.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>