leds-lp8788: fix a parent device in _probe()

The lp8788-keyled is a platform driver of lp8788-mfd. The platform
device is allocated when mfd_add_devices() is called in lp8788-mfd.
On the other hand, 'lp->dev' is the i2c client device.

Therefore, this 'platform_device' is a proper parent device in case
of resource managed mem alloc, registering led device and device
kernel messages.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>