i.MX28: Lower the amount of blocks transfered in one DMA cycle

Some MMC cards, like my ancient 32.0MB SanDisk RS-MMC cards had issue if b_max
was set to 0x40 and DMA was enabled. Lower this value to 0x20, which allows
these cards to work too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>