greybus: protocol: warn on bad deregistration

A protocol should be deregistered exactly once when the protocol module
is being unloaded. This means that protocol deregister will never be
called with active users as we take a module reference when looking up a
protocol.

Remove comment suggesting that we could one day forcefully stop a user
of a protocol, and issue a big warning if a protocol is deregistered
more than once or at some other time than during module unload (e.g.
with active users).

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