net: l2: ethernet: lldp: prevent slist corruption

LLDP start timer is executed when iface is up
at startup, then once more when iface up event
is received.

When iface is up before net_lldp_init call, then multiple
entries are added to the lldp_ifaces related to the
same iface.

That leads to continuous loop execution within
lldp_tx_timeout.

This commit checks if lldp tx_timer_start is set and
doesn't append new entry into the slist.
The tx_timer_start is set at startup and reset on
down event.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>