pm: Fix to prevent incorrect forever timeout

K_TICKS_FOREVER is defined as -1. Guard logic has been added for the case
when `ticks - exit_latency_ticks == -1` to prevent sys_clock_set_timeout()
from incorrectly setting a forever timeout.

Signed-off-by: Serhiy Katsyuba <serhiy.katsyuba@intel.com>