cpupower: bench: parse.c: Fix several minor errors

Resolved several minor errors in prepare_config() and made some additional improvements.
Earlier, the risk of file stream that was not closed. Misuse of strncpy, and the use of strncmp with strlen that makes it pointless.
I also check that sscanf has been successful, otherwise continue to the next line. And minimized the use of magic numbers.

This was found using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>