iio: adc: ad9467: fix reset gpio handling

[ Upstream commit 76f02853 ]

The reset gpio was being handled with inverted polarity. This means that
as far as gpiolib is concerned we were actually leaving the pin asserted
(in theory, this would mean reset). However, inverting the polarity in
devicetree made things work. Fix it by doing it the proper way and how
gpiolib expects it to be done.

While at it, moved the handling to it's own function and dropped
'reset_gpio' from the 'struct ad9467_state' as we only need it during
probe. On top of that, refactored things so that we now request the gpio
asserted (i.e in reset) and then de-assert it. Also note that we now use
gpiod_set_value_cansleep() instead of gpiod_direction_output() as we
already request the pin as output.

Fixes: ad679712

 ("iio: adc: ad9467: add support AD9467 ADC")
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231207-iio-backend-prep-v2-1-a4a33bc4d70e@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>