net: mscc: ocelot: properly reject destination IP keys in VCAP IS1

An attempt is made to warn the user about the fact that VCAP IS1 cannot
offload keys matching on destination IP (at least given the current half
key format), but sadly that warning fails miserably in practice, due to
the fact that it operates on an uninitialized "match" variable. We must
first decode the keys from the flow rule.

Fixes: 75944fda ("net: mscc: ocelot: offload ingress skbedit and vlan actions to VCAP IS1")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>