tools/power turbostat: Fix warning upon failed /dev/cpu_dma_latency read

Previously a failed read of /dev/cpu_dma_latency erroneously complained
turbostat: capget(CAP_SYS_ADMIN) failed, try "# setcap cap_sys_admin=ep ./turbostat

This went unnoticed because this file is typically visible to root,
and turbostat was typically run as root.

Going forward, when a non-root user can run turbostat...
Complain about failed read access to this file only if --debug is used.

Signed-off-by: Len Brown <len.brown@intel.com>