iio: adc: ad7091r8: Fix error code in ad7091r8_gpio_setup()

There is a copy and paste error so it accidentally returns ->convst_gpio
instead of ->reset_gpio.  Fix it.

Fixes: 0b76ff46

 ("iio: adc: Add support for AD7091R-8")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/fd905ad0-6413-489c-9a3b-90c0cdb35ec9@moroto.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>