au1xmmc: raise segment size limit.

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>