[bug report] IIO: ADC: add stm32 DFSDM support for PDM microphone

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Arnaud Pouliquen,

The patch eca949800d2d: "IIO: ADC: add stm32 DFSDM support for PDM
microphone" from Jan 10, 2018, leads to the following static checker
warning:

	drivers/iio/adc/stm32-dfsdm-adc.c:555 stm32_dfsdm_audio_dma_buffer_done()
	warn: potential shift truncation.  '0xffffff00 (0,256-4294967040) << 8'

drivers/iio/adc/stm32-dfsdm-adc.c
   551          while (available >= indio_dev->scan_bytes) {
   552                  u32 *buffer = (u32 *)&adc->rx_buf[adc->bufi];
   553  
   554                  /* Mask 8 LSB that contains the channel ID */
   555                  *buffer = (*buffer & 0xFFFFFF00) << 8;

Is this shift, correct?  The comment just mentions that we're masking
out the low bits.

   556                  available -= indio_dev->scan_bytes;
   557                  adc->bufi += indio_dev->scan_bytes;
   558                  if (adc->bufi >= adc->buf_sz) {
   559                          if (adc->cb)
   560                                  adc->cb(&adc->rx_buf[old_pos],
   561                                           adc->buf_sz - old_pos, adc->cb_priv);
   562                          adc->bufi = 0;
   563                          old_pos = 0;
   564                  }
   565          }
   566          if (adc->cb)
   567                  adc->cb(&adc->rx_buf[old_pos], adc->bufi - old_pos,
   568                          adc->cb_priv);
   569  }

regards,
dan carpenter