kni: fix build on Linux 4.19

kni: fix build on Linux 4.19

The build error observed with Linux kernel 4.19 when KNI ethtool
support enabled (CONFIG_RTE_KNI_KMOD_ETHTOOL=y)

.../build/build/kernel/linux/kni/kni_ethtool.c:193:3:
   error: ‘struct ethtool_ops’ has no member named ‘get_settings’;
  .get_settings  = kni_get_settings,
   ^~~~~~~~~~~~

.../build/build/kernel/linux/kni/kni_ethtool.c:194:3:
   error: ‘struct ethtool_ops’ has no member named ‘set_settings’;
  .set_settings  = kni_set_settings,
   ^~~~~~~~~~~~

With kernel 4.19 ethtool_ops `get_settings` & `set_settings` are
replaced with `get_link_ksettings` & `set_link_ksettings`
Commit 9b3004953503 ("ethtool: drop get_settings and set_settings callbacks")

This fix practically removes `get_settings` & `set_settings` support
for the kernel versions that have the new ethtool_ops without
implementing the new ones.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>