netdev-linux: Don't set ethtool flags if flag is already set on netdev

netdev-linux: Don't set ethtool flags if flag is already set on netdev

Check if ethtool flags is already set on a netdev, before trying to set it.

This patch works around issues with some older verison of ethernet drivers,
which tend to reset the NIC when call to disable LRO is made, even if LRO is
already disable on that NIC. NIC reset is not desirable in OVS upgrade scenario
as it causes extended downtime.

Signed-off-by: Anoob Soman <anoob.soman@citrix.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>