Staging: iio: adis16400: fix some minor issues and clean-up

1. move adis16400_spi_read_burst() to adis16400_ring.c since it is only
   called there
2. add the lost calling to adis16400_self_test()
3. codes cleanup

Signed-off-by: Barry Song <21cnbao@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>