net: sgi: ioc3-eth: allocate space for desc rings only once

Memory for descriptor rings are allocated/freed, when interface is
brought up/down. Since the size of the rings is not changeable by
hardware, we now allocate rings now during probe and free it, when
device is removed.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>