#72 Correct documentation of LinkGetVariables lvMTU

[Andrew Roughan]

The documentation of LinkGetVariables lvMTU is incorrect. The offset is documented as +14 but it should be +16. This value has been incorrectly documented since inception.

Thankfully, the equates files have had the correct value, so there has been no software impact AFAIK

Reported by Ewen Wannop.