net: dsa: qca8k: add support for phy read/write with mgmt Ethernet

Use mgmt Ethernet also for phy read/write if availabale. Use a different
seq number to make sure we receive the correct packet.
On any error, we fallback to the legacy mdio read/write.

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