Nodemgr's ignore_drivers variable is exposed as a module load parameter
(therefore also as a sysfs attribute below /sys/module) and additionally
as an attribute below /sys/bus/ieee1394.  Since the latter is writable,
make the former writable too.

Note, the bus's attribute ignore_drivers is only relevant to newly added
units, not to present or suspended or resuming units.  Those have their
own attribute ignore_driver.
Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: NBen Collins <bcollins@ubuntu.com>