drivers: usb_c: Properly detect a disconnect while operating as a Source

While operating as a Source, a disconnect can be missed
if using an E-Marker cable that presents an Ra. This change
detects a disconnect as follows, while operating as a Source:

CC1     CC2
-----------
OPEN    OPEN ----> Nothing attached
OPEN    Ra   ----> Powered cable without Sink attached
Ra      OPEN ----> Powered cable without Sink attached

Signed-off-by: Sam Hurst <sbh1187@gmail.com>