charger-dock: Change the order of the registration to avoid NPE

NPE will occur if calling switch_set_state() in dock_external_power_changed()
before the registration of swtich driver

Bug: 31085909
Change-Id: Iaea1c40d0ff73967dce2b66e5f7ca61b79c658bb
Signed-off-by: Devin Kim <dojip.kim@lge.com>