cpuidle: menu: Optimize bucket assignment when next_timer_ns equals KTIME_MAX

cpuidle: menu: Optimize bucket assignment when next_timer_ns equals KTIME_MAX

Directly assign the last bucket value instead of calling which_bucket()
when next_timer_ns equals KTIME_MAX, the largest possible value that
always falls into the last bucket.

This avoids unnecessary calculations and enhances performance.

Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>
Link: https://patch.msgid.link/20250405135308.1854342-1-quic_zhonhan@quicinc.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>