lp8788-charger: Fix a parent device in kernel messages

Use 'platform_device' in kernel messages rather than i2c client device
node. lp8788_update_charger_params() needs additional argument, 'pdev'.
Then, remove unnecessary lp8788 private data in lp8788_irq_register().

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>