ieee1394: raw1394: narrow down the state_mutex protected region

Refactor the ioctl dispatcher in order to move a fraction of it out of
the section which is serialized by fi->state_mutex.  This is not so much
about performance but more about self-documentation:  The mutex_lock()/
mutex_unlock() calls are now closer to the data accesses which the mutex
protects, i.e. to the iso_state switch.

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