staging: greybus: light: remove KERNEL_VERSION checks

No need to support older kernel versions in the Greybus Light driver, so
remove the checks as needed, we can now rely on all of the correct LED
core apis being present. And compile only if flash and v4l2 flash is
reachable.

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