net: ETHOC should depend on HAS_DMA

When building for Sun 3:

drivers/net/ethoc.c:1091: undefined reference to `dma_free_coherent'
drivers/built-in.o: In function `ethoc_probe':
drivers/net/ethoc.c:965: undefined reference to `dma_alloc_coherent'
drivers/net/ethoc.c:1063: undefined reference to `dma_free_coherent'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>