tools/power/x86/intel-speed-select: Process mailbox read error for core-power

Some older kernels don't support reading core-power status. In that case
mailbox command fails. So, display core-power status as "unknown" instead
of supported.

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