net: dsa: microchip: use collision based back pressure mode

Errata DS80000758 states that carrier sense back pressure mode can cause
link down issues in 100BASE-TX half duplex mode. The datasheet also
recommends to always use the collision based back pressure mode.

Fixes: b987e98e ("dsa: add DSA switch driver for Microchip KSZ9477")
Signed-off-by: Enguerrand de Ribaucourt <enguerrand.de-ribaucourt@savoirfairelinux.com>
Reviewed-by: Woojung Huh <Woojung.huh@microchip.com>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>