Don't use magic numbers in aes_ocb_cipher()

Lots of references to 16 replaced by AES_BLOCK_SIZE. Also a few other style
tweaks in that function

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2275)
(cherry picked from commit 0ba5a9ea)