firewire: fw-sbp2: fix refcounting

Since patch "fw-sbp2: use an own workqueue (fix system responsiveness)"
increased parallelism between fw-sbp2 and fw-core, it was possible that
fw-sbp2 didn't release the SCSI device when the FireWire device was
disconnected.

This happened if sbp2_update() ran during sbp2_login(), because a bus
reset occurred during sbp2_login().  The sbp2_login() work would [try
to] reschedule itself because it failed due to the bus reset, and it
would _not_ drop its reference on the target.  However, sbp2_update()
would schedule sbp2_login() too before sbp2_login() rescheduled itself
and hence sbp2_update() would take an additional reference.  And then
we would have one reference too many.

The fix is to _always_ drop the reference when leaving the sbp2_login()
work.  If the sbp2_login() work reschedules itself, it takes a
reference, but only if it wasn't already rescheduled by sbp2_update().

Ditto in the sbp2_reconnect() work.

The resulting code is actually simpler than before:  We _always_ take
a reference when successfully scheduling work.  And we _always_ drop
a reference when leaving a workqueue job.  No exceptions.

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