Staging: iio: add some unlocks to raw_read() functions

This code was cut and pasted in several places.  It's missing some
unlocks on error.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>