gpg: Merge two functions in cipher-aead.c

* g10/cipher-aead.c (set_nonce, set_additional_data): Merge into ...
(set_nonce_and_ad): new function.
(write_auth_tag): Print error message here.
(do_flush): Rename var newchunk to finalize.

There is no need to have separate functions here. We should also
print a error message for writing the final tag.

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