net: ethernet: mtk_eth_soc: increase MAX_DEVS to 3

This is a preliminary patch to add MT7988 SoC support since it runs 3
macs instead of 2.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/3563e5fab367e7d79a7f1296fabaa5c20f202d7a.1690246066.git.daniel@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>