staging:iio:ad7606 replaces IIO_ST macro with explicit entries to struct scan_type

IIO_ST is going away as it is a pain to maintain. Note that this driver
currently ensures all fields are converted to the native endianness
to keep things consistent across the different bus types.

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