iio: ad5592r: Drop surplus GPIO header

This driver uses all the modern GPIO APIs from
<linux/gpio/driver.h> and <linux/gpio/consumer.h> so
just drop the unused legacy header <linux/gpio.h>.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>