iio: adc: at91_adc: const-ify some driver data

The main intent is to get rid of the cast for the void-pointer returned by
of_device_get_match_data().

This requires const-ifying the 'caps' and 'registers' references on the
at91_adc_state struct.

The caps can be obtained also from the old platform_data (in the
at91_adc_probe_pdata() function), but that cast is not touched in this
patch, since the old platform_data should be removed/cleaned-away.
Also, that cast deals with converting a kernel_ulong_t type to a pointer.
So, updating that cast doesn't yield any benefit.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930135048.11530-3-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91_adc.c

@@ -202,7 +202,7 @@ struct at91_adc_state {
 	struct mutex		lock;
 	u8			num_channels;
 	void __iomem		*reg_base;
-	struct at91_adc_reg_desc *registers;
+	const struct at91_adc_reg_desc *registers;
 	u32			startup_time;
 	u8			sample_hold_time;
 	bool			sleep_mode;
@@ -214,7 +214,7 @@ struct at91_adc_state {
 	u32			res;		/* resolution used for convertions */
 	bool			low_res;	/* the resolution corresponds to the lowest one */
 	wait_queue_head_t	wq_data_avail;
-	struct at91_adc_caps	*caps;
+	const struct at91_adc_caps	*caps;
 
 	/*
 	 * Following ADC channels are shared by touchscreen:
@@ -550,7 +550,7 @@ static int at91_adc_configure_trigger(struct iio_trigger *trig, bool state)
 {
 	struct iio_dev *idev = iio_trigger_get_drvdata(trig);
 	struct at91_adc_state *st = iio_priv(idev);
-	struct at91_adc_reg_desc *reg = st->registers;
+	const struct at91_adc_reg_desc *reg = st->registers;
 	u32 status = at91_adc_readl(st, reg->trigger_register);
 	int value;
 	u8 bit;
@@ -876,7 +876,7 @@ static int at91_adc_probe_dt(struct iio_dev *idev,
 	if (!node)
 		return -EINVAL;
 
-	st->caps = (struct at91_adc_caps *)of_device_get_match_data(&pdev->dev);
+	st->caps = of_device_get_match_data(&pdev->dev);
 
 	st->use_external = of_property_read_bool(node, "atmel,adc-use-external-triggers");