greybus: connection: clean up svc-connection creation

Move SVC-connection creation to its own helper.

Note that the connection_create host-driver callback is really
unrelated to the SVC connection, and will be removed by a later patch.
It is is included for now as the connection_destroy callback is
currently made from the SVC-connection-destroy helper.

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