network: fix LLDP field type in Interface Varlink IDL

network: fix LLDP field type in Interface Varlink IDL

sd_lldp_tx_describe() returns a single object (the LLDP TX configuration),
but the IDL declared LLDP as SD_VARLINK_ARRAY|SD_VARLINK_NULLABLE. This
caused server-side validation failures ("Field 'LLDP' should be an array,
but it is of type 'object'") whenever networkctl status was called on an
interface with LLDP TX active.

Also fix the field comment: the LLDP field represents the transmit
configuration, not received neighbors.

Follow-up for dd2934d44e2c9cd1a92ae0fd6806985c4bc031e6.