psa: Unused key_bits is OK

When MD or CMAC are disabled, let the compiler know that it is OK that
`key_bits` is set but not used by casting `key_bits` to `(void)`.

library/psa_crypto.c

@@ -1869,6 +1869,7 @@ static psa_status_t psa_mac_setup( psa_mac_operation_t *operation,
     else
 #endif /* MBEDTLS_MD_C */
     {
+        (void) key_bits;
         status = PSA_ERROR_NOT_SUPPORTED;
     }