s390x/tcg: Always use MMU_USER_IDX for CONFIG_USER_ONLY

Although we basically ignore the index all the time for CONFIG_USER_ONLY,
let's simply skip all the checks and always return MMU_USER_IDX in
cpu_mmu_index() and get_mem_index().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>