ieee1394: sbp2: safer agent reset in error handlers

The scsi_host_template's eh_abort_handler and eh_device_reset_handler
are allowed to sleep.  Use this to run sbp2_agent_reset in the more
reliable mode which returns _after_ its write transaction was finished.

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