tools/power turbostat: cleanup 'automatic_cstate_conversion_probe()'

The 'automatic_cstate_conversion_probe()' function has a too long 'if'
statement, convert it to a 'switch' statement in order to improve code
readability a bit.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>