staging:iio:lis3l02dq: General cleanup

As Arnd observed, things are clearner if we pass iio_dev into read and write fucntions.

Now uses st for lis3l02dq_state everywhere.

Other bits of trivial tidying.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>