iio: adis: Switch sampling frequency attr to core support.

By using the info_mask_shared_by_all element of the channel spec, acce
to the sampling frequency becomes available to in kernel users of the
driver.  It also shortens and simplifies the code.

This particular conversion was made more complicated by the shared library
and the fact that a number of the drivers do not actually have support for
setting or reading the sampling frequency.  The hardware, in those cases
investigated supports it. It's just never been implemented.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>