openvpn: fix handling option auth_retry.

As reported in ticket #19104, auth_retry takes a <type> argument with 3
choices: none, nointeract, interact.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45960 3c298f89-4303-0410-b956-a3cf2f4a3e73