driver core: Correct parameter check for API device_for_each_child_reverse_from()

device_for_each_child_reverse_from() checks (!parent->p) for its
parameter @parent, and that is not consistent with other APIs of
its cluster as shown below:

device_for_each_child_reverse_from() // check (!parent->p)
device_for_each_child_reverse() // check (!parent || !parent->p)
device_for_each_child() // same above
device_find_child() // same above

Correct the API's parameter @parent check by (!parent || !parent->p).

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/r/20250105-class_fix-v6-5-3a2f1768d4d4@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>