Staging: iio: adis16300: fix some minor issues and clean-up

1. add delay between spi transfers
2. move burst read to ring function
3. clean-up

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>