openvswitch: Immediately exit on error in ovs_vport_cmd_set().

It is an error to try to change the type of a vport using the set
command. However, while we check that this is an error, we still
proceed to allocate memory which then gets freed immediately.
This stops processing after noticing the error, which does not
actually fix a bug but is more correct.

Signed-off-by: Jesse Gross <jesse@nicira.com>