ath79: apply Engenius ECB1750 style to OpenMesh MR900 RGMII cfg

ath79: apply Engenius ECB1750 style to OpenMesh MR900 RGMII cfg

The OpenMesh MR900 is a modified version of the Exx900/Exx1750 family.
These devices are shipped with an AR803x PHY and had various problems with
the delay configuration in ar71xx. These problems are now in the past [1]
and parts of the delay configuration should now be done in the PHY only.

Just switch to the configuration of the ECB1750 to have an already well
tested configuration for ath79 with the newer kernel versions.

[1] https://github.com/openwrt/openwrt/pull/3505#issuecomment-716050292

Reported-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Sven Eckelmann <sven@narfation.org>