Rip out the old semi-broken support for DMA channel linkage,
in preparation for replacing it with something more capable.

Performance is fine if CONFIG_MMC_BLOCK_BOUNCE is enabled,
although that bounce buffer wastes memory and CPU time.
(In fact it's probably faster than trying to use the dma
chaining code this patch removes.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>