iio:adc:ltc2497 drop of_match_ptr protection

iio:adc:ltc2497 drop of_match_ptr protection

This prevents the driver being used with ACPI PRP0001 based
bindings. Also switch include to mod_devicetable.h which devices
the used id structure.

Note this is most about removing something I don't want want cut
and paste into new driver rather than any thought that this particular
driver will be used in an ACPI system (though it might!)

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