greybus: lights: default value for v4l2 flash controls

V4l2 flash will return erro ERANGE if val(which is the default value)
is not defined. Just set it to the max value reported by the module.

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