[SCSI] 3ware 9000: handle use_sg != 0 for emulated commands

The attached patch updates the driver for the 3ware 9000 series to do
the following:

- Correctly handle single sgl's with use_sg = 1.

This is needed with the latest scsi-block-2.6 merge otherwise the 3w-9xxx
driver will not work.  I tested the patch James sent a few weeks back to fix
this, and it had a bug where the request_buffer was accessed in
twa_scsiop_execute_scsi_complete() when it was invalid.  This is a corrected
variation of that patch.

Signed-off-by: Adam Radford <linuxraid@amcc.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>