net: broadcom: rename BCM4908 driver & update DT binding

compatible string was updated to match normal naming convention so
update driver as well

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>