net: cls_u32: be more strict about skip-sw flag for knodes

Return an error if user requested skip-sw and the underlaying
hardware cannot handle tc offloads (or offloads are disabled).
This patch fixes the knode handling.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>