usb: phy: Factor out the usb charger initialization

Factor out the guts of usb charger initialization into usb_charger_init()
function, to make the usb_add_extcon() only do the extcon related things.
Meanwhile we also should initialize the USB charger before registering
the extcon device, in case the extcon notification was issued earlier than
usb charger initialization.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>