taprio: Add missing policy validation for flags

netlink policy validation for the 'flags' argument was missing.

Fixes: 4cfd5779 ("taprio: Add support for txtime-assist mode")
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>