ice: Use pf instead of vsi-back

Many times in our functions we have a local variable pf, which is
equivalent to vsi->back. Just use pf consistently instead of vsi->back
where available.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>