staging:iio:adis16260: Remove sampling_frequency_available attribute

The values presented here are completely bogus. Also ..._available attributes
are supposed to be used for properties that only support discrete sets of
values, but we accept continuous values and will round to the next supported
frequency.

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