add cast to fix IAR compiler warnings

[Cube707]

Description

We use the IAR compiler to build mbedtls for our embedded project. This compiler throws a warning about these two lines:

mbedtls_ssl_handshake_set_state(ssl, (ssl->state + 1));
                                           ^
"D:\proj\sinamics-m\firmware\lib\mbedtls\library\ssl_misc.h",1361  Error[Pe188]: 
          enumerated type mixed with another type

As we tread all warning as errors, we currently have to apply a patch.

Adding the casts fixes the issue.

Changelog

* Fix IAR compiler warnings.