media: i2c: isl7998x: Use right include

This driver is using GPIO descriptors but uses the legacy
include header <linux/gpio.h>. Fix it by including the
intended <linux/gpio/consumer.h>.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>