bonding: Fix check for ethtool get_link operation support

Since commit 2c60db037034 ('net: provide a default dev->ethtool_ops')
all devices have a non-null ethtool_ops.  Test only
dev->ethtool_ops->get_link in both places where we care.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>