greybus: connection: silence warning on unbound protocols

If a protocol was not successfully created, we can't drop the refcount
on it. This might happen for example if the connection fails to bind a
protocol.

Silences a warning on cleanup.

Signed-off-by: Perry Hung <perry@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>