iio: imu: st_lsm6dsx: move irq related definitions in irq_config

Group irq related definition in irq_config structure in
st_lsm6dsx_settings. This is a preliminary patch to move
OpenDrain/Active low registers in st_lsm6dsx_settings.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>