firewire: core: fix unstable I/O with Canon camcorder

Regression since commit 10389536, "firewire: core: check for 1394a
compliant IRM, fix inaccessibility of Sony camcorder":

The camcorder Canon MV5i generates lots of bus resets when asynchronous
requests are sent to it (e.g. Config ROM read requests or FCP Command
write requests) if the camcorder is not root node.  This causes drop-
outs in videos or makes the camcorder entirely inaccessible.
https://bugzilla.redhat.com/show_bug.cgi?id=633260



Fix this by allowing any Canon device, even if it is a pre-1394a IRM
like MV5i are, to remain root node (if it is at least Cycle Master
capable).  With the FireWire controller cards that I tested, MV5i always
becomes root node when plugged in and left to its own devices.

Reported-by: Ralf Lange
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: <stable@kernel.org> # 2.6.32.y and newer