mv643xx_eth: fix PPC DMA breakage

After 2.6.29, PPC no more admits passing NULL to the dev parameter of
the DMA API. The result is a BUG followed by solid lock-up when the 
mv643xx_eth driver brings an interface up. The following patch makes 
the driver work on my Pegasos again; it is mostly a search and replace 
of NULL by mp->dev->dev.parent in dma allocation/freeing/mapping/unmapping
functions.

Signed-off-by: Gabriel Paubert <paubert@iram.es>
Acked-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>