firewire: fw-sbp2: expose module parameter for workarounds

On rare occasions, the ability to set one of the workaround flags at
runtime may save the day.

People who experience I/O errors with firewire-sbp2 while the old sbp2
driver worked for them should try workarounds=1 and report to the devel
mailinglist whether that improves things.  Firewire-sbp2 defaults to the
SCSI stack's maximum transfer size per command, while sbp2 limits them
to 128 kBytes.  Flag 1 accomplishes just that.

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