net: dsa: qca8k: set internal delay also for sgmii

QCA original code report port instability and sa that SGMII also require
to set internal delay. Generalize the rgmii delay function and apply the
advised value if they are not defined in DT.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>