Fix a typo in the SSL_get_max_early_data() declarations

SSL_get_max_early_data() recently added by 3fc8d856

 ("Construct the
ticket_early_data_info extension", 2017-02-17) is supposed to take an
SSL, but it doesn't.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3113)