net: ti: am65x-cpsw: fix mac tx internal delay for rgmii-rxid mode

Now AM65x CPSW2G driver will disable MAC TX internal delay for PHY
interface mode "rgmii-rxid" which is incorrect. Hence, fix it by keeping
default value (enabled) for MAC TX internal delay when "rgmii-rxid"
interface mode is selected.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>