tools/power/x86/intel-speed-select: Handle open() failure case

Add handling for open() failure case to make sure a valid file
descriptor is passed to dup().

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>