firewire: cdev: fix IR multichannel event documentation

State more precisely when fw_cdev_event_iso_interrupt_mc is sent.
While the comment tried to reflect an amibuity in the OHCI
specification, there is only one method that is useful in practice,
and this also matches all the hardware implementations I've tested.

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