firewire: ohci: prevent iso completion callbacks after context stop

To prevent the iso packet callback from being called after
fw_iso_context_stop() has returned, make sure that the
context's tasklet has finished executing before that.

This fixes access-after-free bugs that have so far been
observed only in the upcoming snd-firewire-speakers driver,
but can theoretically also happen in the firedtv driver.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>