net: ethernet: mtk_eth_soc: enable XDP support just for MT7986 SoC

Disable page_pool/XDP support for MT7621 SoC in order fix a regression
introduce adding XDP for MT7986 SoC. There is no a real use case for XDP
on MT7621 since it is a low-end cpu. Moreover this patch reduces the
memory footprint.

Tested-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Fixes: 23233e57 ("net: ethernet: mtk_eth_soc: rely on page_pool for single page buffers")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/2bf31e27b888c43228b0d84dd2ef5033338269e2.1663074002.git.lorenzo@kernel.org


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