[SCSI] 3w-9xxx fix bug in sgl loading

This small patch fixes a bug in the 3w-9xxx driver where it would load
an invalid sgl address in the ioctl path even if request length was zero.

Signed-off-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>