mv643xx_eth: use longer DMA bursts

The mv643xx_eth driver is limiting DMA bursts to 32 bytes, while
using the largest burst size (128 bytes) gives a couple percentage
points performance improvement in throughput tests, and the docs
say that the 128 byte default should not need to be changed, so
use 128 byte bursts instead.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>