i40e: Cleanup i40e_vlan_rx_register

We used to use the function i40e_vlan_rx_register as a way to hook
into the now defunct .ndo_vlan_rx_register netdev hook. This was
removed but we kept the function around because we still used it
internally to control enabling or disabling of VLAN stripping.

As pointed out in upstream review, VLAN stripping is only used in a
single location and the previous function is quite small, just inline
it into i40e_restore_vlan() rather than carrying the function
separately.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>