Use the standard WARN_ON instead.
If a small kernel is desired, WARN_ON can be disabled globally.

Also remove SSB_DEBUG. Besides WARN_ON it only adds a tiny debug check.
Include this check unconditionally.

Signed-off-by: Michael Buesch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>