netdevsim: don't overwrite read only ethtool parms

Ethtool ring feature has _max_pending attributes read-only.
Set only read-write attributes in nsim_set_ringparam.

This patch is useful, if netdevsim device is set-up using NetworkManager,
because NetworkManager sends 0 as MAX values, as it is pointless to
retrieve them in extra call, because they should be read-only. Then,
the device is left in incosistent state (value > MAX).

Fixes: a7fc6db099b5 ("netdevsim: support ethtool ring and coalesce settings")
Signed-off-by: Filip Pokryvka <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/netdevsim/ethtool.c

···
 {
 	struct netdevsim *ns = netdev_priv(dev);
 
-	memcpy(&ns->ethtool.ring, ring, sizeof(ns->ethtool.ring));
+	ns->ethtool.ring.rx_pending = ring->rx_pending;
+	ns->ethtool.ring.rx_jumbo_pending = ring->rx_jumbo_pending;
+	ns->ethtool.ring.rx_mini_pending = ring->rx_mini_pending;
+	ns->ethtool.ring.tx_pending = ring->tx_pending;
 	return 0;
 }