turbostat: Check return value of fscanf

Some systems declare fscanf with the warn_unused_result attribute.  On
such systems, turbostat generates the following warnings:

turbostat.c: In function 'get_core_id':
turbostat.c:1203:8: warning: ignoring return value of 'fscanf', declared with attribute warn_unused_result [-Wunused-result]
turbostat.c: In function 'get_physical_package_id':
turbostat.c:1186:8: warning: ignoring return value of 'fscanf', declared with attribute warn_unused_result [-Wunused-result]
turbostat.c: In function 'cpu_is_first_core_in_package':
turbostat.c:1169:8: warning: ignoring return value of 'fscanf', declared with attribute warn_unused_result [-Wunused-result]
turbostat.c: In function 'cpu_is_first_sibling_in_core':
turbostat.c:1148:8: warning: ignoring return value of 'fscanf', declared with attribute warn_unused_result [-Wunused-result]

Fix these by checking the return value of those four calls to fscanf.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Len Brown <len.brown@intel.com>