xl2tpd: fix invoking xl2tpd-control

Upstream commit 90884c62 ("xl2tpd-control refactoring") introduced in
1.3.16 changed command names

The l2tp protocol handler part was from @danvd in pull request
openwrt/packages#13866

Fixes f07319d6

 ("xl2tpd: bump to version 1.3.16")

Ref: https://github.com/openwrt/packages/pull/13866
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>