iio: vcnl4000: Factorize data reading and writing.

Factorize data reading in vcnl4000_measure into a vcnl4000_read_data
function. Also add a vcnl4000_write_data function.

Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>