openvswitch: Fix ovs_vport_cmd_del return value on success

If the pointer does not represent an error then the PTR_ERR macro may still
return a nonzero value. The fix is the same as in ovs_vport_cmd_set.

Signed-off-by: Rich Lane <rlane@bigswitch.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>