firewire: fw-sbp2: wait for completion of fetch agent reset

Like the old sbp2 driver, wait for the write transaction to the
AGENT_RESET to complete before proceeding (after login, after reconnect,
or in SCSI error handling).

There is one occasion where AGENT_RESET is written to from atomic
context when getting DEAD status for a command ORB.  There we still
continue without waiting for the transaction to complete because this
is more difficult to fix...

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