cpuidle: use first valid target residency as poll time

Commit 259231a0 ("cpuidle: add poll_limit_ns to cpuidle_device
structure") changed, by mistake, the target residency from the first
available sleep state to the last available sleep state (which should
be longer).

This might cause excessive polling.

Fixes: 259231a0 ("cpuidle: add poll_limit_ns to cpuidle_device structure")
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>