[settings] Use list_first_entry() when unregistering child settings

[settings] Use list_first_entry() when unregistering child settings

Unregistering a child settings block can have almost arbitrary
effects, due to the call to apply_settings().  Avoid potentially
dereferencing a stale pointer by using list_first_entry() rather than
list_for_each_entry_safe() to iterate over the list of child settings.

Signed-off-by: Michael Brown <mcb30@ipxe.org>