i40e: Add helper to access main VSI

Add simple helper i40e_pf_get_main_vsi(pf) to access main VSI
that replaces pattern 'pf->vsi[pf->lan_vsi]'

Reviewed-by: Michal Schmidt <mschmidt@redhat.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>