iio: frequency: m2k-dac: fix match function type

Error is:
```
drivers/iio/frequency/m2k-dac.c:507:7: error: passing argument 4 of ‘bus_find_device’ from incompatible pointer type [-Werror=incompatible-pointer-types]
       dds_dev_match);
```

The second parameter just needs a const specifier.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

drivers/iio/frequency/m2k-dac.c

@@ -605,7 +605,7 @@ static int m2k_dac_alloc_channel(struct platform_device *pdev,
 	return 0;
 }
 
-static int dds_dev_match(struct device *dev, void *data)
+static int dds_dev_match(struct device *dev, const void *data)
 {
 	int ret;