Rework mbedtls_test_param_failed_get_state_buf()

Rework mbedtls_test_param_failed_get_state_buf() and
its documentation.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index a963da9..fff065a 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -211,7 +211,7 @@
 
 void* mbedtls_test_param_failed_get_state_buf( void )
 {
-    return &param_failed_ctx.state[0];
+    return &param_failed_ctx.state;
 }
 
 void mbedtls_test_param_failed_reset_state( void )