drivers: serial: ra8_sci_b: adjust return usage in void functions

For code clarity, this commit adjusts the use of `return` statements
in functions with a void return type by:
- Remove unnecessary `return` statements when
  they don't affect control flow.

Signed-off-by: Pisit Sawangvonganan <[email protected]>

Author: ndrs-pst

drivers/serial/uart_renesas_ra8_sci_b.c

@@ -458,15 +458,15 @@ static inline void async_rx_disabled(const struct device *dev)
 	struct uart_event event = {
 		.type = UART_RX_DISABLED,
 	};
-	return async_user_callback(dev, &event);
+	async_user_callback(dev, &event);
 }
 
 static inline void async_request_rx_buffer(const struct device *dev)
 {
 	struct uart_event event = {
 		.type = UART_RX_BUF_REQUEST,
 	};
-	return async_user_callback(dev, &event);
+	async_user_callback(dev, &event);
 }
 
 static inline void async_rx_ready(const struct device *dev)