bcm63xx_enet: do not write to random DMA channel on BCM6345

The DMA controller regs actually point to DMA channel 0, so the write to
ENETDMA_CFG_REG will actually modify a random DMA channel.

Since DMA controller registers do not exist on BCM6345, guard the write
with the usual check for dma_has_sram.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>