net: enetc: check the index of the SFI rather than the handle

We should check whether the current SFI (Stream Filter Instance) table
is full before creating a new SFI entry. However, the previous logic
checks the handle by mistake and might lead to unpredictable behavior.

Fixes: 888ae5a3

 ("net: enetc: add tc flower psfp offload driver")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>