Fix typo setting up certificate masks

The certificate masks are used to select which ciphersuite we are going to
use. The variables |emask_k| and |emask_a| relate to export grade key
exchange and authentication respecitively. The variables |mask_k| and
|mask_a| are the equivalent versions for non-export grade. This fixes an
instance where the two usages of export/non-export were mixed up. In
practice it makes little difference since it still works!

Reviewed-by: Richard Levitte <levitte@openssl.org>