iio: imu: adis16400: move trigger handler into adis16400_core

The trigger handler for the ADIS16400 is very different from the generic
one in the ADIS library. Keeping it in a separate file won't make much
sense once the update_scan_mode function will be made more generic and
moved into the ADIS library.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>