net: dsa: microchip: ksz8795: move cpu_select_interface to extra function

This patch moves the cpu interface selection code to a individual
function specific for ksz8795. It will make it simpler to customize the
code path for different switches supported by this driver.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>