Use mbedtls_ct_condition_t in mpi_core_check_zero

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index b56be0a..92c8d47 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -662,11 +662,11 @@
  * \param[in] A   The MPI to test.
  * \param limbs   Number of limbs in \p A.
  *
- * \return        0 if `A == 0`
- *                non-0 (may be any value) if `A != 0`.
+ * \return        MBEDTLS_CT_FALSE if `A == 0`
+ *                MBEDTLS_CT_TRUE  if `A != 0`.
  */
-mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
-                                                size_t limbs);
+mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
+                                                      size_t limbs);
 
 /**
  * \brief          Returns the number of limbs of working memory required for