extcon: axp288: Set EXTCON_USB when EXTCON_CHG_USB_SDP was set

According to the documentation, we should set the EXTCON_USB when
one SDP charger connector was reported.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>