Remove controller dependency from fragmenter

Only ISO packets use transmit_downward, so the event_type can be
removed.

Pass the ISO buffer size into the fragmenter; it doesn't need to be
fetched from the controller on every packet.

Bug: 301661850
Test: atest net_test_hci
Flag: EXEMPT, no logical change
Change-Id: I17c468e67045b145418578e2e74ed23391019e47