Change mbedtls_mpi_core_mla() to be constant time

Signed-off-by: Waleed Elmelegy <waleed.elmelegy@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index 1a3e0b9..b43be30 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -489,7 +489,7 @@
 
     while (excess_len--) {
         *d += c;
-        c = (*d < c);
+        c = mbedtls_ct_mpi_uint_if(mbedtls_ct_uint_lt(*d, c), 1, 0);
         d++;
     }