platform/x86: intel-uncore-freq: Conditionally create attribute for read frequency

When the current uncore frequency can't be read, don't create attribute
"current_freq_khz" as any read will fail later. Some user space
applications like turbostat fail to continue with the failure. So, check
error during attribute creation.

Fixes: 414eef27

 ("platform/x86/intel/uncore-freq: Display uncore current frequency")
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231004181915.1887913-1-srinivas.pandruvada@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>