Staging: iio: meter: ade7854-i2c: code style improvements

Code reformatting based on checkpatch.pl with --strict:
Comparison to NULL rewritten as !indio_dev

Signed-off-by: Tolga Ceylan <tolga.ceylan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>