power: qpnp-smbcharger: return UNKNOWN in case of no charger present

POWER_SUPPLY_TYPE_USB_DCP type is returned in read_usb_type() if no
charger is connected, which might cause APSD to unnecessarily rerun.
To fix it, POWER_SUPPLY_TYPE_UNKNOWN should be returned instead.

A bug is also fixed by returning right after IDEV_STS register
read error in this function .

CRs-fixed: 963745
Change-Id: Ia399e05fe8d0ad13c2f1f272c6737e8798177174
Signed-off-by: Harry Yang <harryy@codeaurora.org>