iio:adc:ti-adc12138: Switch to generic firmware properties and drop of_match_ptr

This enables using the driver with other firmware types such
as ACPI via PRP0001.

Also part of a general attempt to move IIO drivers over to generic
properties to avoid opportunities for cut and paste.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>