iio: event: NULL-ify IIO device's event_interface ref during unregister

iio: event: NULL-ify IIO device's event_interface ref during unregister

Though we know that the iio_device_unregister_eventset() call is followed
by the free-ing of the IIO device object, we should not make this
assumption in the iio_device_unregister_eventset() function. It should
allow for the clean unregistering of the event-set, allowing a re-register
should we decide to implement this at some point later.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200921103156.194748-2-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>