iio: adc: at91: add missing const qualifier

Add missing const qualifier for variable which induces a compiler
warning.

Fixes e5a69c1 ("iio: adc: at91: fix wrong channel number in triggered buffer mode")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>

Author: ehristev

drivers/iio/adc/at91_adc.c

@@ -248,7 +248,7 @@ static irqreturn_t at91_adc_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *idev = pf->indio_dev;
 	struct at91_adc_state *st = iio_priv(idev);
-	struct iio_chan_spec *chan;
+	struct iio_chan_spec const *chan;
 	int i, j = 0;
 
 	for (i = 0; i < idev->masklength; i++) {