Ethernet switch and MDIO are grouped using "simple-bus". It's not
allowed to use "ethernet-switch" node name as it isn't a switch. Replace
it with "bus".

Fixes: 527a3ac9 ("arm64: dts: broadcom: bcm4908: describe internal switch")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>