[bsp/at32] 1.fixed f402 uart7/8 pierph clock source error, 2.fixed compiling error of drv_usart_v2.c with no dma

bsp/at32/libraries/AT32F402_405_Firmware_Library/drivers/src/at32f402_405_usart.c

@@ -115,7 +115,7 @@ void usart_init(usart_type* usart_x, uint32_t baud_rate, usart_data_bit_num_type
   crm_clocks_freq_type clocks_freq;
   uint32_t apb_clock, temp_val;
   crm_clocks_freq_get(&clocks_freq);
-  if((usart_x == USART1) || (usart_x == USART6) ||  (usart_x == UART7) || (usart_x == UART8))
+  if((usart_x == USART1) || (usart_x == USART6))
   {
     apb_clock = clocks_freq.apb2_freq;
   }

bsp/at32/libraries/rt_drivers/drv_usart_v2.c

@@ -398,7 +398,9 @@ static rt_ssize_t at32_transmit(struct rt_serial_device *serial, rt_uint8_t *buf
 
     if(instance->uart_dma_flag & RT_DEVICE_FLAG_DMA_TX)
     {
+#ifdef RT_SERIAL_USING_DMA
         _uart_dma_transmit(instance, buf, size);
+#endif
         return size;
     }