staging/iio: iio_hwmon: Use device tree node name for hwmon name attribute

So far, all instances of iio_hwmon set their hwmon name attribute
to "iio_hwmon", which is not very descriptive. Set it to the device tree
node name if available, and only revert to iio_hwmon otherwise.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>