Notice that it is not necessary to evaluate _STA in find_child_checks()
if the device is expected to have children, but there are none, so
move the children check to the front of the function.

Also notice that FIND_CHILD_MIN_SCORE can be returned right away if
_STA is missing, so make the function do so.

Finally, replace the ternary operator in the return statement argument
with an if () and a standalone return which is somewhat easier to
follow.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>