config.h: Don't use arithmetical exp for SSL_DTLS_MAX_BUFFERING

The functions requires_config_value_at_least and requires_config_value_at_most
only work with numerical constants.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 70dd4be2..1cdff71 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3016,7 +3016,7 @@
  * DTLS handshake message reassembly and future message buffering.
  *
  */
-//#define MBEDTLS_SSL_DTLS_MAX_BUFFERING             ( 2 * 16384 )
+//#define MBEDTLS_SSL_DTLS_MAX_BUFFERING             32768
 
 //#define MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME     86400 /**< Lifetime of session tickets (if enabled) */
 //#define MBEDTLS_PSK_MAX_LEN               32 /**< Max size of TLS pre-shared keys, in bytes (default 256 bits) */