media: et8ek8: select V4L2_FWNODE

v4l2_async_register_subdev_sensor_common() is only provided when
CONFIG_V4L2_FWNODE is enabled, otherwise we get a link failure:

drivers/media/i2c/et8ek8/et8ek8_driver.o: In function `et8ek8_probe':
et8ek8_driver.c:(.text+0x884): undefined reference to `v4l2_async_register_subdev_sensor_common'

This adds a Kconfig 'select' statement like all the other users of
this interface have.

Fixes: d8932f38 ("media: et8ek8: Add support for flash and lens devices")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>