Make the chip info structure data optional by providing reasonable
defaults.  Improve corresponding documentation, and highlight the drawback
of not providing explicit chipselect control.

DMA can determine appropriate dma_burst_size and thresholds automatically
so use DMA even if dma_burst_size is not specified.
Signed-off-by: NVernon Sauder <VernonInHand@gmail.com>
Reviewed-by: NNed Forrester <nforrester@whoi.edu>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>