mmc: tmio: name 0xd8 as CTL_DMA_ENABLE

This reflects at least the current usage of this register
and I think it improves the readability of the code ever so slightly.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>