lldp: fix parsing of TLV length

lldp: fix parsing of TLV length

tlv_packet_read_bytes() and tlv_packet_read_string() returned the
wrong length when called after other functions which modify the offset
in the container.

In other words, if the TLV data length is X and we do a
tlv_packet_read_u8(), a subsequent tlv_packet_read_bytes() should
return a length of (X - 1).