odp-util: Fix converting masked VLAN from flow.

When converting the VLAN from a flow to an ODP key, the processing logic
would always store the VLAN ethertype.  However, when handling a mask,
it should be a mask, not an ethertype.  And since we don't support
bit-wise masking of the ethertype, just make it an exact-match mask.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>