ixgbe: Clean up redundancy in hw_enc_features

Clean up minor redundancy in the setting of hw_enc_features that
makes it appears that X550 uniquely has more encapsulation features
than other devices. The driver only supports one more feature, so
make it look that way. No longer set NETIF_F_SG since that is set
by the register_netdev call. Thanks to Alex Duyck for noticing this
slight confusion.

Reported-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>