MFC r335982, r335985, r335988-r335989

r335982:
Fix an out-of-bounds array access... the irq data for teardown is in two
arrays, as elements 0 and 1 of one array and elements 1 and 2 of the other.
Run the loop 0..1 instead of 1..2 and use named constants to offset into
one of the arrays.

PR: 229508

r335985:
Remove a test and early-out which just can't possibly be right. It causes
detach() to do nothing if attach() succeeded, which is the opposite of
what's needed. Also, move device_delete_children() from the end to the
beginning of detach(), so that children won't be trying to make use of the
hardware we're in the process of shutting down.

PR: 229510

r335988:
Add a missing call to usb_bus_mem_free_all() when detaching.

r335989:
Detach all children before beginning to tear down the hardware, instead of
doing it last. Also, remove the local tracking of whether usb's busdma
memory allocation got done, because it's safe to call the free_all
function even if it wasn't.