greybus: svc: clean up CPortFlags handling

Clean up CPortFlags handling and explicitly disable CSD when E2EFC is
enabled (CSD_n is ignored when E2EFC is set).

Note that the bootrom requires E2EFC, CSD, and CSV to all be disabled.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>