mmc: esdhc: Fix DMA_MASK to not break mx25 DMA access

Patch to not set reserved bits in i.MX25 PROCTL register.  DMA stops
working if those bits get set.

Signed-off-by: Wilson Callan <wilson.callan@savantsystems.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>