[SCSI] fix 3ware raid emulated commands

The 3ware emulated commands all expect they are executing in the
use_sg == 0 case, which isn't true either in the block layer rework or
an SG_IO ioctl.

Fix this by adding the correct kmapping of the first element in the sg
list.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>