Raise the DMA block size limit from 2048 bytes to the maximum supported
by the DMA controllers on the chip (64KB on Au1100, 4MB on Au1200).

This gives a very small performance boost and apparently fixes an oops
when MMC-DMA and network traffic are active at the same time.
Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>