Cavium CNN55XX: fix broken default Kconfig entry

Every developer always thinks that _their_ code is so special and
magical that it should be enabled by default.

And most of them are completely and utterly wrong.  That's definitely
the case when you write a specialty driver for a very unsual "security
processor". It does *not* get to mark itself as "default m".

If you solve world hunger, and make a driver that cures people of
cancer, by all means enable it by default.  But afaik, the Cavium
CNN55XX does neither.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>