extcon: intel-cht-wc: Refactor cht_wc_extcon_get_charger()

This is a preparation patch for adding support for registering
a power_supply class device.

Setting usbsrc to "CHT_WC_USBSRC_TYPE_SDP << CHT_WC_USBSRC_TYPE_SHIFT"
will make the following switch-case return EXTCON_CHG_USB_SDP
just as before, so there is no functional change.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>