net: ethernet: mtk_eth_soc: ppe: prevent ppe update for non-mtk devices

Introduce an additional validation to ensure that the PPE index
is modified exclusively for mtk_eth ingress devices.
This primarily addresses the issue related
to WED operation with multiple PPEs.

Fixes: dee4dd10

 ("net: ethernet: mtk_eth_soc: ppe: add support for multiple PPEs")
Signed-off-by: Elad Yifee <eladwf@gmail.com>
Link: https://lore.kernel.org/r/20240623175113.24437-1-eladwf@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>