net: broadcom: BCM4908_ENET should not default to y, unconditionally

Merely enabling compile-testing should not enable additional code.
To fix this, restrict the automatic enabling of BCM4908_ENET to
ARCH_BCM4908.

Fixes: 4feffead ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>