datapath: Update comments about 'OVS_KEY_ATTR_8021Q'.

Commit fea393b1 (datapath: Describe policy for extending flow key,
implement needed changes.) changed the key 'OVS_KEY_ATTR_8021Q' to
'OVS_KEY_ATTR_VLAN' and the size of the attribute structure.  A couple
of comments were missed, so this commit updates them.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>