ice: cleanup vf_id signedness

The vf_id variable is dealt with in the code in inconsistent
ways of sign usage, preventing compilation with -Werror=sign-compare.
Fix this problem in the code by always treating vf_id as unsigned, since
there are no valid values of vf_id that are negative.

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>