staging:iio:adis16400: Remove samplerate_available attribute

While the samplerate supported by the devices which are supported by this driver
is not continuous it supports a wide range, much more than currently listed in
the samplerate_available attribute. Also it accepts all values written to the
samplerate attribute and will round-up them to the nearest supported sample
rate. So remove the samplerate_available attribute.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>