Re-enable posted writes for status FIFO.

Besides bringing back a very minor bandwidth tweak from Linux 2.6.15.x
and older, this also fixes an interoperability regression since 2.6.16:

   http://bugzilla.kernel.org/show_bug.cgi?id=6356
   (sbp2: scsi_add_device failed. IEEE1394 HD is not working anymore.)
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Vanei Heidemann <linux@javanei.com.br>
Tested-by: Martin Putzlocher <mputzi@gmx.de> (chip type unconfirmed)
Signed-off-by: Linus Torvalds <torvalds@osdl.org>