ath79: disable delays on AT803X config init

ath79: disable delays on AT803X config init

Until upstream commit 6d4cd041f0af("net: phy: at803x: disable delay
only for RGMII mode"), delays were not disabled on driver probe
for the Atheros AR803x PHYs, although the RX delay is enabled on
soft and hard reset.

In addition, the TX delay setting is retained on soft-reset.

This patch disables both delays on config init to align the behavior
with kernel 5.1 and higher. It can be safely dropped with kernel 5.1.

Signed-off-by: David Bauer <mail@david-bauer.net>