ethoc: Fix warning in ethoc_init_ring().

Get rid of the pointless back-and-forth casting of dev->mem_start
from long to pointer back to long again.

Also fixes a warning reported by Stephen Rothwell:

drivers/net/ethoc.c: In function 'ethoc_init_ring':
drivers/net/ethoc.c:302: warning: assignment makes integer from pointer without a cast

Signed-off-by: David S. Miller <davem@davemloft.net>