i40e: Remove umem from VSI

As current implementation of netdev already contains and provides
umems for us, we no longer have the need to contain these
structures in i40e_vsi.

Refactor the code to operate on netdev-provided umems.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>