Normalize tls1_enc return values.

The distinction between publicly and non-publicly invalid is barely acted upon
and slightly silly now that the CBC padding check has been folded into
EVP_AEAD.

Change-Id: Idce4b9b8d29d624e3c95243a147265d071612127
Reviewed-on: https://boringssl-review.googlesource.com/2980
Reviewed-by: Adam Langley <agl@google.com>