ath79: use "fixed-layout" for Embedded Wireless devices

ath79: use "fixed-layout" for Embedded Wireless devices

Those devices have Ethernet interfaces using base MAC address increased
by 0x40 in the 3rd indexed byte (00:00:00:FF:00:00). To describe that we
were using a custom (downstream) "mac-address-increment-byte" property.

The same result can be achieved by using "mac-base" with a properly
adjusted offset value (0x40 << 16). It may be not pretty but it should
work without custom property or downstream kernel patch to support it.

Cc: Ansuel Smith <ansuelsmth@gmail.com>
Cc: Catrinel Catrinescu <cc@80211.de>
Cc: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rosen Penev <rosenp@gmail.com>