[serial-v2]fix the data type error

components/drivers/include/drivers/serial_v2.h

@@ -178,7 +178,7 @@ struct rt_uart_ops
     int (*putc)(struct rt_serial_device *serial, char c);
     int (*getc)(struct rt_serial_device *serial);
 
-    rt_size_t (*transmit)(struct rt_serial_device       *serial,
+    rt_ssize_t (*transmit)(struct rt_serial_device       *serial,
                                  rt_uint8_t             *buf,
                                  rt_size_t               size,
                                  rt_uint32_t             tx_flag);

components/drivers/serial/serial_v2.c

@@ -455,6 +455,7 @@ static rt_ssize_t _serial_fifo_tx_blocking_nbuf(struct rt_device        *dev,
 {
     struct rt_serial_device *serial;
     struct rt_serial_tx_fifo *tx_fifo = RT_NULL;
+    rt_ssize_t rst;
 
     RT_ASSERT(dev != RT_NULL);
     if (size == 0) return 0;
@@ -476,14 +477,14 @@ static rt_ssize_t _serial_fifo_tx_blocking_nbuf(struct rt_device        *dev,
 
     tx_fifo->activated = RT_TRUE;
     /* Call the transmit interface for transmission */
-    serial->ops->transmit(serial,
-                          (rt_uint8_t *)buffer,
-                          size,
-                          RT_SERIAL_TX_BLOCKING);
+    rst = serial->ops->transmit(serial,
+                                (rt_uint8_t *)buffer,
+                                size,
+                                RT_SERIAL_TX_BLOCKING);
     /* Waiting for the transmission to complete */
     rt_completion_wait(&(tx_fifo->tx_cpt), RT_WAITING_FOREVER);
 
-    return size;
+    return rst;
 }
 
 /**