usb: pd: force disconnect upon unexpected SRC->SNK transition

While in Source mode, a notification that a source is attached
is unexpected. In this case, force a disconnect in order to
clean up state and turn off VBUS and Vconn.

Change-Id: Ie576feeb7622376e8cb0dd8454b626b3b24b190b
Signed-off-by: Jack Pham <jackp@codeaurora.org>