phy: atheros: Explicitly disable RGMII delays

To eliminate any doubts about the out-of-reset value of the PHY, that
the driver previously relied on.

If bisecting shows that this commit breaks your board you probably have
a wrong PHY interface mode. You probably want the
PHY_INTERFACE_MODE_RGMII_RXID or PHY_INTERFACE_MODE_RGMII_ID mode.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>