dhcpv6-pd: T421: fix unknown lvalue 'StartLimitIntervalSec' warning

StartLimitIntervalSec is part of the [Unit] definition and not the [Service]
definition [1]. This caused the following warning message:

systemd[1]: /lib/systemd/system/dhcp6c@.service:12: Unknown lvalue
  'StartLimitIntervalSec' in section 'Service', ignoring

This error has been introduced via commit 992d356 ("dhcpv6-pd: T421: workaround
for non existing interfaces").

[1]: https://www.freedesktop.org/software/systemd/man/systemd.unit.html