greybus: lights: add v4l2 flash operations

We do not implement any of the v4l2 flash operations, as the default
ones are ok for now, however the init needs anything define, if not it
will return an error. So, just define it and have an error free v4l2
flash init.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>