net/ethernet: ARM_AT91_ETHER should depend on HAS_DMA

If NO_DMA=y:

drivers/built-in.o: In function `at91ether_start':
drivers/net/ethernet/cadence/at91_ether.c:49: undefined reference to `dma_alloc_coherent'
drivers/net/ethernet/cadence/at91_ether.c:60: undefined reference to `dma_free_coherent'
drivers/built-in.o: In function `at91ether_interrupt':
drivers/net/ethernet/cadence/at91_ether.c:250: undefined reference to `dma_unmap_single'
drivers/built-in.o: In function `at91ether_start_xmit':
drivers/net/ethernet/cadence/at91_ether.c:169: undefined reference to `dma_map_single'
drivers/built-in.o: In function `at91ether_close':
drivers/net/ethernet/cadence/at91_ether.c:145: undefined reference to `dma_free_coherent'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>