PM: runtime: Simplify __rpm_get_callback()

Simplify __rpm_get_callback() slightly by returning as soon as the return
value is known.

No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>