firewire: core: add tracepoints events for flushing of isochronous context

It is helpful to trace the flushing of isochronous context when the core
function is requested them by both in-kernel unit drivers and userspace
applications.

This commit adds some tracepoints events for the aim.

Link: https://lore.kernel.org/r/20240623220859.851685-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>