[SCSI] bnx2fc: Tx BDs cache in write tasks

When there is a single BD for the entire data to be transmitted, use the BD
inside the SGL context and set the cached SGE indication in the task context

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>