ssh-opencl will produce false negatives for SSH keys supported only by the CPU format

[solardiz]

See #5742. Those test vectors are currently within #ifdef CPU_FORMAT, but this merely hides the problem.

After an easy change in valid() it can detect and complain.

../run/john --skip --format=ssh-opencl Novo.txt 
[ssh-opencl] cipher value of 5 (if EC_KEY_WITH_AES256) is not yet supported with OpenCL!
Using default input encoding: UTF-8
No password hashes loaded (see FAQ)