cpuidle: ACPI: do not overwrite name and description of C0

Fix a bug that leads to showing the name and description of C-state C0
as "<null>" in sysfs after the ACPI C-states changed (e.g. after AC->DC
or DC->AC
transition).

The function poll_idle_init() in drivers/cpuidle/driver.c initializes the
state 0 during cpuidle_register_driver(), so we better do not overwrite it
again with '\0' during acpi_processor_cst_has_changed().

Signed-off-by: Thomas Schlichter <thomas.schlichter@web.de>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: 3.13+ <stable@vger.kernel.org> # 3.13+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>