tcp: unloading a module that is set to default should error.

[rrs]

Summary

I just discovered that the return of the EBUSY error was incorrectly
rigged so that you could unload a CC module that was set to default.
Its supposed to be an EBUSY error. Make it so.

Test Plan

make sure that after loading a module and setting it
to default that we get a EBUSY return on unload. And if
you remove the default then the module unload works.