psa_utils: Make fallthrough clear for compilers

Silence a compiler warning about implicit fallthrough by using a comment
format the compiler understand to mean that the fallthrough is
intentional.

  In file included from library/cipher.c:63:0:
  include/mbedtls/psa_util.h: In function ‘mbedtls_psa_translate_cipher_mode’:
  include/mbedtls/psa_util.h:91:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
               if( taglen == 0 )
                 ^
  include/mbedtls/psa_util.h:94:9: note: here
           default:
           ^~~~~~~
  cc1: all warnings being treated as errors

  $ gcc --version
  gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

include/mbedtls/psa_util.h

@@ -91,6 +91,7 @@ static inline psa_algorithm_t mbedtls_psa_translate_cipher_mode(
             if( taglen == 0 )
                 return( PSA_ALG_CBC_NO_PADDING );
             /* Intentional fallthrough for taglen != 0 */
+            /* fallthrough */
         default:
             return( 0 );
     }