Fix IPv6 HTTPClient vendor class data

The size of the ClassIdentifier is apparently wrong in the structure. In my
testing it caused the vendor class data to be displayed as bogus
"HTTPClient:Arch00016x:UNDI00300". After correcting the size the vendor class
data is "HTTPClient:Arch00016:UNDI003000" which looks good to me.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>