Fix SSL_CIPHER_is_AESGCM.

It was checking algorithm_mac rather than algorithm_enc.
(Coincidentally, it gave the right answer if you compiled out the
ChaCha20 ciphers since SSL_AES128GCM and SSL_AEAD shared a value.)

Change-Id: I17047425ef7fabb98969144965d8db9528ef8497
Reviewed-on: https://boringssl-review.googlesource.com/5850
Reviewed-by: Adam Langley <agl@google.com>