greybus: connection: add CSD connection flag

Add CSD connection flag that can be specified when allocating a
connection to enable Controlled Segment Dropping in favour of E2EFC
which is enabled by default.

Note that most connections are expected to have E2EFC enabled.

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