subsys/pm: Use correct timeout API

The z_set_timeout_expiry() function is a wrapper provided in the
timeout subsystem that has some corrections for timelicing.  But the
suspend code doesn't care; by definition there are no active threads
that might be timeslicing at the point where the OS is suspending.
Use the lower-level timer driver API instead.

(z_set_timeout_expiry() is also about to disappear)

Signed-off-by: Andy Ross <andyross@google.com>