ar71xx: Use PHY fixups for Open Mesh MR1750

ar71xx: Use PHY fixups for Open Mesh MR1750

The delays of PHY/MAC on the MR1750 are done by u-boot and OpenWrt in
different ways. u-boot only modifies the ETH_CFG of the QCA955x based on
the link speed. But OpenWrt can only modify the PHY delays based on the
link speed.

This can lead to communication problems when u-boot initializes the ETH_CFG
for a specific link speed (e.g. 10BASE-T) but then OpenWrt the sets the PHY
delays to an incompatible value.

Instead reset the ETH_CFG delay bits of the QCA955x to a specific value and
only rely on the AT803x PHY settings.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Backport of r49031