ice: Add ice_get_main_vsi to get PF/main VSI

There are multiple places where we currently use ice_find_vsi_by_type
to get the PF (a.k.a. main) VSI. The PF VSI by definition is always
the first element in the pf->vsi array (i.e. pf->vsi[0]). So instead
add and use a new helper function ice_get_main_vsi, which just returns
pf->vsi[0].

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>