[MMC] sdhci: more DMA capabilities tests

Properly test for controller interface to see if it's DMA capable.  As many
controllers are misconfigured in this regard, also add debug parameters to
force DMA support either way.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>