dpif-netdev: Proper error handling in do_add_port().

dpif-netdev: Proper error handling in do_add_port().

This fixes multiple error path mistakes in do_add_port, none of which
has been a problem in practice so far. This change will make it easier
for a following commit to return in case of error.

Also, this removes an unneeded special case for tunnel ports.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Tested-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Mark Kavanagh <mark.b.kavanagh@intel.com>