staging:iio:adc: AD7780: Use private data space from iio_allocate_device + trivial fixes

Use private data space from iio_allocate_device
Drop dev_data in favour of iio_priv()
Fix typo in gpio name
Fix error return path, free gpio
Make scale_uv type unsigned long

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>