st/mesa: MAX_VARYING is the max supported number of patch varyings, not min

st/mesa: MAX_VARYING is the max supported number of patch varyings, not min

This fixes
GL45-CTS.tessellation_shader.tessellation_shader_tessellation.max_in_out_attributes
on nouveau. We only support 30 patch varyings (as 2 vec4 slots end up
being used for tess level settings), but were getting 32 exposed.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 7d3f9ed71c71637a91ecf15f50dbe7578a65d57e)