MX28: Fix up the MMC driver DMA mode

The DMA mode didn't properly configure the DMA_ENABLE bit in CTRL1.
Also, it was using SSP0 DMA channel for all SSP devices.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>