Try to write dual-phase retry protocol limits to BUSY_TIMEOUT register.
- The dual-phase retry protocol is optional to implement, and if not 
  supported, writes to the dual-phase portion of the register will be
  ignored. We try to write the original 1394-1995 default here.
- In the case of devices that are also SBP-3-compliant, all writes are 
  ignored, as the register is read-only, but contains single-phase retry of
  15, which is what we're trying to set for all SBP-2 device anyway, so this
  write attempt is safe and yields more consistent behavior for all devices.

See section 8.3.2.3.5 of the 1394-1995 spec, section 6.2 of the SBP-2 spec,
and section 6.4 of the SBP-3 spec for further details.
Signed-off-by: Jarod Wilson <jwilson@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>