iio:chemical:vz89x: Drop of_match_ptr protection and use generic fw accessors

This change allow the driver to be used with ACPI PRP0001 and removes
an antipattern that I want to avoid being copied into new IIO drivers.

The handling of match_data uses a different approach as
device_get_match_data() doesn't distinguish between no match, and
a match but with NULL data.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20200910173242.621168-30-jic23@kernel.org