ethtool: __ethtool_set_sg: check for function pointer before using it

__ethtool_set_sg does not check if dev->ethtool_ops->set_sg is defined
which can result in a NULL pointer dereference when ethtool is used to
change SG settings for drivers without SG support.

Signed-off-by: Roger Luethi <rl@hellgate.ch>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>