ice: Get VF VSI instances directly via PF

This patch changes how we get VF VSIs instances. Instead of relying on
mailbox virtual channel message to retrieve VSI, it is more reliable
getting it directly via VF object in PF data structure.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@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>