cpuidle: poll_state: Fix default time limit

The default time is declared in units of microsecnds,
but is used as nanoseconds, resulting in significant
accounting errors for idle state 0 time when all idle
states deeper than 0 are disabled.

Under these unusual conditions, we don't really care
about the poll time limit anyhow.

Fixes: 800fb34a ("cpuidle: poll_state: Disregard disable idle states")
Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>