iio: adc: nau7802: Remove redundant of_match_ptr

nau7802_dt_ids is always compiled in. Hence of_match_ptr is not
needed.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>