radv: Fix handling of attribs 16-31.
We really need to update the enum for consistency, but that involves a bunch of GL & bitfield work which is error-prone, so since this is a fix for stable lets do the simple things. Confirmed that nothing in radv/aco/nir/spirv uses MAX_VERT_ATTRIB except the one thing I bumped. CC: mesa-stable Reviewed-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Part-of: <mesa/mesa!6120>