i40e: Consolidate checks whether given VSI is main

In the driver code there are 3 types of checks whether given
VSI is main or not:
1. vsi->type ==/!= I40E_VSI_MAIN
2. vsi ==/!= pf->vsi[pf->lan_vsi]
3. vsi->seid ==/!= pf->vsi[pf->lan_vsi]->seid

All of them are equivalent and can be consolidated. Convert cases
2 and 3 to case 1.

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>