i40evf: Fix turning TSO, GSO and GRO on after

This patch fixes the problem where each MTU change turns TSO,
GSO and GRO on from off state.

Now when TSO, GSO or GRO is turned off, MTU change does not
turn them on.

Signed-off-by: Paweł Jabłoński <pawel.jablonski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>