i40e: validate VSI param from VFs

Validate that the VF has sent us a valid VSI index before actually using
that index. Without this code, a malicious or buggy VF driver could
panic the host by sending an invalid index into the VSI array.

Change-ID: I66a177687a0dcc281ec83e714d3813d70d18c8b4
Reported-by: Nick Nunley <nicholas.d.nunley@intel.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>