leds: mc13783: Fix MC13892 keypad led access

Fix the register access shift argument calculation introduced with
commit a59ce658 ("leds: leds-mc13783: Add MC34708 LED support")
and re-enable access to the "keypad" led for MC13892 MFC devices.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>