[driver][serial] V1]: fix correct data loss logic when RX ring buffer is full

In the serial ISR (`rt_hw_serial_isr`), the previous logic for handling a full RX FIFO was flawed. When the buffer was filled, it would increment `get_index` (`get_index += 1`).

This had two negative consequences:
1.  It effectively discarded the oldest byte of data prematurely.
2.  It reduced the usable capacity of a buffer of size N to N-1. For example, a 64-byte buffer could only ever hold 63 readable bytes after becoming full.

This patch corrects the behavior by implementing a standard overwriting ring buffer strategy. When the buffer is full, the logic is changed to `get_index = put_index`.

This ensures that:
- When new data arrives, it correctly overwrites the oldest data.
- The `get_index` is advanced along with the `put_index`, correctly marking the new start of the buffer.
- The full N-byte capacity of the buffer is utilized, always storing the N most recent bytes.

This change resolves the unexpected data loss and makes the buffer behavior correct and predictable.

components/drivers/serial/dev_serial.c

@@ -1463,9 +1463,8 @@ void rt_hw_serial_isr(struct rt_serial_device *serial, int event)
                 /* if the next position is read index, discard this 'read char' */
                 if (rx_fifo->put_index == rx_fifo->get_index)
                 {
-                    rx_fifo->get_index += 1;
+                    rx_fifo->get_index = rx_fifo->put_index;
                     rx_fifo->is_full = RT_TRUE;
-                    if (rx_fifo->get_index >= serial->config.bufsz) rx_fifo->get_index = 0;
 
                     _serial_check_buffer_size();
                 }