firewire: Grab dev->sem when doing the update callback.

This serializes the update callback with the probe and remove callback from
the driver core and prevents remove from being called while update is
running for the same device.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>