Simplify mmc_davinci_send_dma_request() by relying on constraints
provided through its call stack.  Document those constraints a bit
better.

Lift the unnecessary constraint that blocks not cross scatterlist
segments.  If we handle multi-segment transfers at all, there's no
need for such a restriction.

(This is a code shrink that *should* change no behavior.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>