All modern chips support and need the 10_100 bit set for supporting jumbo
frames on 10/100 ports, so instead of enabling it only for 583XX enable
it for everything except bcm63xx, where the bit is writeable, but does
nothing.

Tested on BCM53115, where jumbo frames were dropped at 10/100 speeds
without the bit set.

Fixes: 6ae5834b ("net: dsa: b53: add MTU configuration support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>