iio: adc: ad7923: use device-managed function for triggered buffer

Converts the iio_triggered_buffer_setup() call to its device-managed
counterpart.
With this, the error handling routine in the ad7923_probe() function
becomes unnecessary as well as the iio_triggered_buffer_cleanup() call.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/fe46a45caaa788f333d78367968272de728f4a6e.1616966903.git.lucas.p.stankus@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>