firewire: debug interrupt events

This adds debug printks for asynchronous transmission and reception and
for self ID reception.  They can be enabled at module load time, and at
runtime via /sys/module/firewire_ohci/parameters/debug.

Signed-off-by: Jarod Wilson <jwilson@redhat.com>

Also added:  Logging of interrupt event codes and of cancelled AT
packets.

The code now depends on a Kconfig variable.  This makes it easier to
build firewire-ohci without the feature or to make it an option in the
future.  The variable is currently hidden and always on.

This feature inflates firewire-ohci.ko by 7 kB = 27% on x86-64 and by
4 kB = 23% on i686.

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