enic: fixes for review items from Ben Hutchings

Fixes for review items from Ben Hutchings:
 - use netdev->net_stats rather than private net_stats
 - use ethtool op .get_sset_count rather than .get_stats_count
 - err out if setting Tx/Rx csum or TSO using ethtool and setting is
   not enabled for device.
 - pass in jiffies + constant to round_jiffies
 - return err if new MTU is out-of-bounds

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>