net: dsa: setup master ethtool after dsa_ptr

DSA overrides the master's ethtool ops so that we can inject its CPU
port's statistics. Because of that, we need to setup the ethtool ops
after the master's dsa_ptr pointer has been assigned, not before.

This patch setups the ethtool ops after dsa_ptr is assigned, and
restores them before it gets cleared.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>