iio: imu: adis16400 switch sampling frequency attr to core support.

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

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