datapath: Allow GRE64 to use flow based tunneling.

There's no reason why GRE64 can't be configured using flow based
tunneling.  Since that's the direction we are moving, everything
should support that.  Once this special casing is removed, it's
possible to unify the tunnel parameter parsing code since both
GRE and CAPWAP are the same.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>