net: dsa: microchip: add common duplex and flow control function

This patch add common function for configuring the Full/Half duplex and
transmit/receive flow control. KSZ8795 uses the Global control register
4 for configuring the duplex and flow control, whereas all other KSZ9477
based switch uses the xMII Control 0 register.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>