net/ethernet: NET_CALXEDA_XGMAC should depend on HAS_DMA

If NO_DMA=y:

drivers/built-in.o: In function `xgmac_xmit':
drivers/net/ethernet/calxeda/xgmac.c:1102: undefined reference to `dma_mapping_error'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>