i40e: Remove back pointer from i40e_hw structure

The .back field placed in i40e_hw is used to get pointer to i40e_pf
instance but it is not necessary as the i40e_hw is a part of i40e_pf
and containerof macro can be used to obtain the pointer to i40e_pf.
Remove .back field from i40e_hw structure, introduce i40e_hw_to_pf()
and i40e_hw_to_dev() helpers and use them.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>