lldp: fix parsing of vlan-name attribute

We used to read 3 bytes after the TLV, fix this.

Also, check that string length is at most 32 bytes as specified in
figure E.3 of IEEE 802.1AB-2009.

Fixes: 18133ea1

https://bugzilla.redhat.com/show_bug.cgi?id=1652210
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