userspace: Fix thread index type in z_thread_perms_all_clear()

The type for the thread index returned by thread_index_get() must be
casted to int when comparing with (-1). Directly using uintptr_t is
breaking the ARMv8 implementation where where the check (index != -1) is
verified also when no thread index is returned.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>