net: ethernet: mtk_eth_soc: fix flow block refcounting logic

Since we call flow_block_cb_decref on FLOW_BLOCK_UNBIND, we also need to
call flow_block_cb_incref for a newly allocated cb.
Also fix the accidentally inverted refcount check on unbind.

Fixes: 502e84e2

 ("net: ethernet: mtk_eth_soc: add flow offloading support")
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/20230330120840.52079-1-nbd@nbd.name
Signed-off-by: Jakub Kicinski <kuba@kernel.org>