lldpd: implement lldp_policy parameter

lldpd: implement lldp_policy parameter

For certain lldp_class scenarios (2 & 3) a policy must be set also.
Class 4 is default, although it's good to handle the policy eventuality.

Here, set a default lldp_policy for all lldp_class scenarios. Any
lldp_policy can now be set.

Depends on PR #14584 (which introduced an `if` block)

Tested on 22.03.5, 22.03.6

Signed-off-by: Paul Donald <newtwen@gmail.com>
(cherry picked from commit 497fafb8aea48e764ce0e456f20ab346691e585f)
Link: https://github.com/openwrt/openwrt/pull/15299
Signed-off-by: Robert Marko <robimarko@gmail.com>