tests: ringbuffer: Use defined req_len

Fixes "stored and never read" warning.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
diff --git a/tests/lib/ringbuffer/src/main.c b/tests/lib/ringbuffer/src/main.c
index 55dd2e1..41a26c7 100644
--- a/tests/lib/ringbuffer/src/main.c
+++ b/tests/lib/ringbuffer/src/main.c
@@ -864,8 +864,8 @@
 		zassert_equal(req_len, len, NULL);
 
 		req_len = 2;
-		len = ring_buf_put_claim(&ringbuf_raw, &ptr, 2);
-		zassert_equal(len, 2, NULL);
+		len = ring_buf_put_claim(&ringbuf_raw, &ptr, req_len);
+		zassert_equal(len, req_len, NULL);
 
 		len = ring_buf_put_claim(&ringbuf_raw, &ptr, RINGBUFFER_SIZE);
 		len2 = ring_buf_put_claim(&ringbuf_raw, &ptr, RINGBUFFER_SIZE);