iio: Add IIO_STATIC_CONST_DEVICE_ATTR

Add IIO_STATIC_CONST_DEVICE_ATTR macro for creating an read-only
iio_dev_attr which returns constant value. This macro is intended to be
used when replacing the IIO_CONST_ATTR - attributes for triggered
buffers because the triggered buffer attributes must be of type
iio_dev_attr.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/8dd853dd0ef8eb40cb980cc6f6e7a43166de3afb.1664782676.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/sysfs.h

@@ -97,6 +97,17 @@ struct iio_const_attr {
 	= { .string = _string,						\
 	    .dev_attr = __ATTR(_name, S_IRUGO, iio_read_const_attr, NULL)}
 
+#define IIO_STATIC_CONST_DEVICE_ATTR(_name, _string)				\
+	static ssize_t iio_const_dev_attr_show_##_name(			\
+					struct device *dev,		\
+					struct device_attribute *attr,	\
+					char *buf)			\
+	{								\
+		return sysfs_emit(buf, "%s\n", _string);		\
+	}								\
+	static IIO_DEVICE_ATTR(_name, 0444,				\
+			       iio_const_dev_attr_show_##_name, NULL, 0)
+
 /* Generic attributes of onetype or another */
 
 /**