crypto/tls: enable TLS_FALLBACK_SCSV in server with default max version

Fix TLS_FALLBACK_SCSV check when comparing the client version to the
default max version. This enables the TLS_FALLBACK_SCSV check by default
in servers that do not explicitly set a max version in the tls config.

Change-Id: I5a51f9da6d71b79bc6c2ba45032be51d0f704b5e
Reviewed-on: https://go-review.googlesource.com/1776
Reviewed-by: Adam Langley <agl@golang.org>