backlight: move hp680-bl's probe function to .devinit.text

A pointer to hp680bl_probe is passed to the core via
platform_driver_register and so the function must not disappear when the
.init sections are discarded.  Otherwise (if also having HOTPLUG=y)
unbinding and binding a device to the driver via sysfs will result in an
oops as does a device being registered late.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Kristoffer Ericson <Kristoffer.Ericson@gmail.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>