iio: as3935: Drop GPIO includes

The driver includes <linux/gpio.h> and <linux/of_gpio.h> yet
fails to use symbols from any of the include files, so drop
these includes.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>