usb: pd: Only enable SOP' reception when acting as VCONN source

We only expect to receive SOP' messages if we have enabled
VCONN and are communicating to the cable. Initialize the PD PHY
to enable SOP' reception accordingly during PE_SRC/SNK_Startup
based on whether VCONN is enabled or not.

Change-Id: If08d37a896d706886566ee2d784af50393d9e604
Signed-off-by: Jack Pham <jackp@codeaurora.org>