radv: Only expose subgroup shuffles on VI+.

The current implementation depends on bpermute, which
is VI+.

Fixes: f2c6a550 "radv: enable subgroup capabilities"
Reviewed-by: Daniel Schürmann <daniel.schuermann@campus.tu-berlin.de>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit c2799574)