lib: ring_buf: Fix unused-but-set compilation warnings

For the error codes used only in __ASSERT() statements fix compilation
warnings like:

...
warning: variable 'err' set but not used [-Wunused-but-set-variable]
        int err;
            ^
...

Signed-off-by: Andrei Emeltchenko <[email protected]>

Author: finikorg

lib/os/ring_buffer.c

@@ -71,6 +71,7 @@ uint32_t ring_buf_put(struct ring_buf *buf, const uint8_t *data, uint32_t size)
 
 	err = ring_buf_put_finish(buf, total_size);
 	__ASSERT_NO_MSG(err == 0);
+	ARG_UNUSED(err);
 
 	return total_size;
 }
@@ -139,6 +140,7 @@ uint32_t ring_buf_get(struct ring_buf *buf, uint8_t *data, uint32_t size)
 
 	err = ring_buf_get_finish(buf, total_size);
 	__ASSERT_NO_MSG(err == 0);
+	ARG_UNUSED(err);
 
 	return total_size;
 }
@@ -164,6 +166,7 @@ uint32_t ring_buf_peek(struct ring_buf *buf, uint8_t *data, uint32_t size)
 	/* effectively unclaim total_size bytes */
 	err = ring_buf_get_finish(buf, 0);
 	__ASSERT_NO_MSG(err == 0);
+	ARG_UNUSED(err);
 
 	return total_size;
 }
@@ -214,6 +217,7 @@ int ring_buf_item_put(struct ring_buf *buf, uint16_t type, uint8_t value,
 
 	ret = ring_buf_put_finish(buf, total_size);
 	__ASSERT_NO_MSG(ret == 0);
+	ARG_UNUSED(ret);
 
 	return 0;
 }
@@ -260,6 +264,7 @@ int ring_buf_item_get(struct ring_buf *buf, uint16_t *type, uint8_t *value,
 
 	ret = ring_buf_get_finish(buf, total_size);
 	__ASSERT_NO_MSG(ret == 0);
+	ARG_UNUSED(ret);
 
 	return 0;
 }