i40e: Add helpers to find VSI and VEB by SEID and use them

Add two helpers i40e_(veb|vsi)_get_by_seid() to find corresponding
VEB or VSI by their SEID value and use these helpers to replace
existing open-coded loops.

Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>