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

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.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D33229