greybus: connection: drop unneeded gb_protocol_put() calls

Although a connection records its protocol id when it gets
created, its protocol handler doesn't actually get assigned
until gb_connection_bind_protocol() is called.

In gb_connection_create() there are some error paths in
which a reference to the connection's protocol is released
before the protocol handler has been associated with the
connection.

Get rid of those calls.

As a result, we will never pass a null protocol pointer to
gb_protocol_put().  Add a precautionary warning in that
function in the event that ever occurs.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>