i40e: Use consistent type for vf_id

The driver was all over the place using signed or unsigned types
for vf_id, when it should always be signed.

This fixes warnings of type unsafe comparisons from gcc with W=2.

Change-Id: I2cb681f83d0f68ca124d2e4131e4ac0d9f8a6b22
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>