Register _childdetached methods with drivers that attach children.
Wait to set child references to NULL there, instead of doing that
in the detach method.
Replace many uses of USB_DECLARE_DRIVER() with CFATTACH_DECL2().
unused in this driver's callbacks, so this is an aesthetic rather than
functional change ("ucom0 at uslsa0" rather than "ucom0 at uslsa0
portno -848524548").