Bluetooth: Controller: Fix headroom reservation for ISO

Calling `net_buf_reserve` removed any previous reservation, which
includes the reservation by hci_ipc to hold the H4 type byte. This
resulted in a out-of-bounds net_buf_push in hci_ipc.

This commit also enables asserts in hci_ipc in the audio bsim test,
which reveal the out-of-bounds write.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>