i40evf: Don't enable vlan stripping when rx offload is turned on

With current implementation of i40evf_set_features when user sets
any offload via ethtool we set I40EVF_FLAG_AQ_ENABLE_VLAN_STRIPPING
as a required aq which triggers driver to call
i40evf_enable_vlan_stripping. This shouldn't take place.
This patches fixes it by setting the flag only when VLAN offload
is turned on.

Signed-off-by: Patryk Małek <patryk.malek@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>