cpuidle: Drop unnecessary type cast in cpuidle_poll_time()

The data type of the target_residency_ns field in struct cpuidle_state
is u64, so it does not need to be cast into u64.

Get rid of the unnecessary type cast.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>