hwmon: (iio_hwmon) Use devm functions

Use devm_iio_channel_get_all() to automatically release
channels.

Use devm_hwmon_device_register_with_groups() to
automatically unregister the device.

Signed-off-by: Maxime Roussin-Bélanger <maxime.roussinbelanger@gmail.com>
[groeck: Dropped now unnecessary platform_set_drvdata() and hwmon_dev]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>