iio: light: vcnl4000 use word writes instead of byte writes

The VCNL4200 datasheet says that word read and writes should be used
to access the registers.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Tested-by: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>