staging:iio:adis16400: Fix adis16334 sampling frequency control

Setting the sampling frequency for the adis16334 differs from the other devices.
This patch introduces two new callback functions to the adis16400 chip_info
struct which are used to specify how to read and write the current sample rate.
The patch also introduces the proper implementations for these callbacks for the
adis16334.

Related to this is that the adis16334 has no slow mode and so we do not limit
the SPI clock rate to 300kHz during initialization. The patch adds a new flag
for devices which do have a slow mode.

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