iio:adc:ltc2496: Drop of_match_ptr and use mod_devicetable.h

The macro prevents use of driver with ACPI PRP0001 and
the of_match_id structure is defined in mod_devicetable.h so include
that directly rather than of.h.

Note this is mostly about removing something I don't want cut and
paste into new drivers rather than expectation that this particular
driver will be used on an ACPI platform.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>