net: dsa: Do not check for ethtool_ops validity

This is completely redundant with what netdev_set_default_ethtool_ops()
does, we are always guaranteed to have a valid dev->ethtool_ops pointer,
however, within that structure, not all function calls may be populated,
so we still have to check them individually.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>