staging:iio:ad7280a: Reflect optionality of irq in ABI

Given the irq is optional, let us remove the interfaces related to
events when it is not present.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-13-jic23@kernel.org