iio: accel: kxcjk1013: make sysfs/sampling_frequency_avail dynamic

In preparation for KXTF9 support, make sampling_frequency_avail
attribute dynamic.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>