[SCSI] bfa: Introduced generic address len pair to represent DMA memory chunk.

- Avoid the use of hardware defined structure bfi_sge_s for DMA
  requests in host.

- Defined a generic address len pair to represent a DMA memory chunk
  (bfi_alen_s).

Signed-off-by: Krishna Gudipati <kgudipat@brocade.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>