firewire: sbp2: protect a reference counter properly

The assertion in the comment in sbp2_allow_block() is no longer true.
Or maybe it never was true.  At least now, the sole caller of
sbp2_allow_block(), sbp2_login, can run concurrently to one of
sbp2_unblock()'s callers, sbp2_remove.

sbp2_login is performed by sbp2_logical_unit.work.
sbp2_remove is performed by fw_device.work.
sbp2_remove cancels sbp2_logical_unit.work, but only after it called
sbp2_unblock.

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