iio: vcnl4000: make the driver extendable

There are similar chips in the vcnl4xxx family. The initialization and
communication is a bit different for members of the family, so this
patch makes the driver extendable for different chips.

There is no functional change.

Signed-off-by: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>