It's always one. We don't support other kinds of curves with this framework.
(Curve25519 uses a much simpler API.) This also allows us to remove the
check_pub_key_order logic.
Change-Id: Ic15e1ecd68662b838c76b1e0aa15c3a93200d744
Reviewed-on: https://boringssl-review.googlesource.com/8350
Reviewed-by: Adam Langley <agl@google.com>