tools/power turbostat: replace strncmp with single character compare

Using strncmp for a single character comparison is overly complicated,
just use a simpler single character comparison instead. Also stops
static analyzers (such as cppcheck) from complaining about strncmp on
non-null terminated strings.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>