net: ethernet: mtk_eth_soc: add missing ppe cache flush when deleting a flow

The cache needs to be flushed to ensure that the hardware stops offloading
the flow immediately.

Fixes: 33fc42de

 ("net: ethernet: mtk_eth_soc: support creating mac address based offload entries")
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230330120840.52079-3-nbd@nbd.name
Signed-off-by: Jakub Kicinski <kuba@kernel.org>