vlan/macvlan: fix NULL pointer dereferences in ethtool handlers

[ Upstream commit 7816a0a862d851d0b05710e7d94bfe390f3180e2 ]

Check whether the underlying device provides a set of ethtool ops before
checking for individual handlers to avoid NULL pointer dereferences.

Reported-by: Art van Breemen <ard@telegraafnet.nl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>