mbedtls: STM32F439xI: Don't enable AES acceleration by default

STM32F439xI-family AES hardware acceleration occasionally produces
incorrect output (#4928).

Don't enable AES HW acceleration on STM32F439xI-family targets by
default until issue #4928 is fixed.

Author: Patater

features/mbedtls/targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F439xI/mbedtls_device.h

@@ -20,7 +20,9 @@
 #ifndef MBEDTLS_DEVICE_H
 #define MBEDTLS_DEVICE_H
 
-#define MBEDTLS_AES_ALT
+/* FIXME: Don't enable AES hardware acceleration until issue #4928 is fixed.
+ * (https://github.com/ARMmbed/mbed-os/issues/4928) */
+/* #define MBEDTLS_AES_ALT */
 
 #define MBEDTLS_SHA256_ALT