staging: iio: dac: ad5446: Remove unused sign member from chip specific information structure

Remove unused sign member from chip specific information structure
Fix typos

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>