ftgmac100: Make ring sizes configurable via ethtool

We set an arbitrary max at 1024 since we pre-allocate the actual
descriptor arrays and skb arrays to the full size to keep the
code a bit simpler and avoid allocation failures in the reset
task.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>