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>

drivers/mmc/mxsmmc.c

@@ -406,7 +406,7 @@ int mxsmmc_initialize(bd_t *bis, int id, int (*wp)(int))
 	 */
 	mmc->f_min = 400000;
 	mmc->f_max = mxc_get_clock(MXC_SSP0_CLK + id) * 1000 / 2;
-	mmc->b_max = 0x40;
+	mmc->b_max = 0x20;
 
 	mmc_register(mmc);
 	return 0;