Update the davinci_mmc driver to stop hard-wiring use of
mmc0 DMA channels:  look them up during probe, record them,
and use the recorded values everywhere, not constants.
Add a previously-missing diagnostic if probe() fails.

Also fix a seeming bug in the rarely-used davinci_abort_dma()
call.  It was aborting the wrong channel ... the idle one,
not the active one.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>