iio: adc: at91_adc: add Kconfig dep on the OF symbol and remove of_match_ptr()

iio: adc: at91_adc: add Kconfig dep on the OF symbol and remove of_match_ptr()

This tries to solve a warning reported by the lkp bot:

>> drivers/iio/adc/at91_adc.c:1439:34: warning: unused variable
>> 'at91_adc_dt_ids' [-Wunused-const-variable]
   static const struct of_device_id at91_adc_dt_ids[] = {
                                    ^
   1 warning generated.

This warning has appeared after the AT91_ADC driver compilation has been
enabled via the COMPILE_TEST symbol dependency.

The warning is caused by the 'of_match_ptr()' helper which returns NULL if
OF is undefined. This driver should build only for device-tree context, so
a dependency on the OF Kconfig symbol has been added.
Also, the usage of of_match_ptr() helper has been removed since it
shouldn't ever return NULL (because the driver should not be built for the
non-OF context).

Fixes: 4027860dcc4c ("iio: Kconfig: at91_adc: add COMPILE_TEST dependency to driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930135048.11530-4-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>