ecc: Avoid double conversion to affine coordinates in keygen.

* cipher/ecc.c (nist_generate_key): Add args r_x and r_y.
(ecc_generate): Rename vars.  Convert to affine coordinates only if
not returned by the lower level generation function.

nist_generate_key already needs to convert to affine coordinates to
implement Jivsov's trick. Thus we can return them and avoid calling
it in ecc_generate again.

• Signed-off-by: Werner Koch <wk@gnupg.org>