net: phy: realtek: Change TX-delay setting for RGMII modes only

It's prone to problems if delay is cleared out for other than RGMII
modes. So lets set/clear the TX-delay in the config register only
if actually RGMII-like interface mode is requested. This only
concerns rtl8211f chips.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>