lldpd: set a 30 seconds lower limit to the safeguard timer

lldpd: set a 30 seconds lower limit to the safeguard timer

This timer is a safeguard to refresh information about all networks
interfaces at regular interval, in case there was something wrong with
the event-based refresh.

When using milliseconds-grained tx-interval, this could happen several
times per second, which is not the intended use of the safeguard.

To quote Vincent Bernat:
	"Minimal value of 30 seconds even if we don't have event-based
	refresh (so outside the if). Most people have the event-based
	refresh, so lowering the pace for the others should be pretty
	invisible."