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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6120>