drivers/rapidio/devices/tsi721_dma.c: optimize use of BDMA descriptors

Combine SG entries describing single contiguous memory block into one
Tsi721 BDMA descriptor.  This reduces number of hardware descriptors
required for large data transfers and improves performance on the PCIe
side by reducing number of descriptor fetch requests.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>