iio: addac: stx104: Use define rather than hardcoded limit for write val

The DAC register is 16 bits wide, so the value passed by write_raw()
should be checked against that limit. Rather than hardcoding the 16-bit
maximum value limit, use a define to improve readability and make the
intention of the code clearer. The explicit cast is also avoided by
instead explicitly checking for negative values.
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/4c9f4f1b4a270d133be70c82a091351b531b5e3e.1680790580.git.william.gray@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/addac/stx104.c

@@ -13,6 +13,7 @@
 #include <linux/ioport.h>
 #include <linux/isa.h>
 #include <linux/kernel.h>
+#include <linux/limits.h>
 #include <linux/module.h>
 #include <linux/moduleparam.h>
 #include <linux/mutex.h>
@@ -181,8 +182,7 @@ static int stx104_write_raw(struct iio_dev *indio_dev,
 		return 0;
 	case IIO_CHAN_INFO_RAW:
 		if (chan->output) {
-			/* DAC can only accept up to a 16-bit value */
-			if ((unsigned int)val > 65535)
+			if (val < 0 || val > U16_MAX)
 				return -EINVAL;
 
 			mutex_lock(&priv->lock);