[ch32][bsp] fix warning: rt_size_t to rt_ssize_t

bsp/wch/risc-v/Libraries/ch32_drivers/drv_usart.c

@@ -343,7 +343,7 @@ static int ch32_getc(struct rt_serial_device *serial)
     return ch;
 }
 
-rt_size_t ch32dma_transmit(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
+static rt_ssize_t ch32dma_transmit(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
 {
     return RT_EOK;
 }