ssl_cache: return the error code for mutex failure

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/include/mbedtls/ssl_cache.h b/include/mbedtls/ssl_cache.h
index 41ada54..55dcf77 100644
--- a/include/mbedtls/ssl_cache.h
+++ b/include/mbedtls/ssl_cache.h
@@ -134,7 +134,7 @@
  *
  * \return                0: The cache entry for session with provided ID
  *                           is removed or does not exist.
- *                        1: Internal error.
+ *                        Otherwise: fail.
  */
 int mbedtls_ssl_cache_remove(void *data,
                              unsigned char const *session_id,