Revert "[cyassl]: upgrade to 2.8.0"

Reverts the CyaSSL version bump for now since the update completely broke
trunk building due to incompatible changes in the IO callback API which in
turn breaks the core ustream-ssl package.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38576 3c298f89-4303-0410-b956-a3cf2f4a3e73