au1000_eth: Fix warnings.

Fixed by including <linux/dma-mapping.h>:

  CC      drivers/net/au1000_eth.o
drivers/net/au1000_eth.c: In function 'au1000_probe':
drivers/net/au1000_eth.c:661: warning: implicit declaration of function 'dma_alloc_noncoherent'
drivers/net/au1000_eth.c:802: warning: implicit declaration of function 'dma_free_noncoherent'

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>