iio:adc:cpcap-adc: Drop of_match_ptr protection and use device_get_match_data

Part of a slow effort to avoid OF specific code in IIO.

Whilst the main advantages of this are not likely to be seen in this
particular driver (ACPI support via PRP0001) the change proposed
does make things a bit more maintainable and also ensures that
this particular (now) anti-patern is less likely to be cut and
paste into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>