cpuidle: lpm-levels: Allow exit latencies equal to target latencies

This allows pm_qos votes with, say, 100 us for example to select power
levels with exit latencies equal to 100 us. The extra microsecond of
exit latency doesn't hurt.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>