net: dsa: vsc73xx: Assert reset if iCPU is enabled

Driver allow to use devices with disabled iCPU only.

Some devices have pre-initialised iCPU by bootloader.
That state make switch unmanaged. This patch force reset
if device is in unmanaged state. In the result chip lost
internal firmware from RAM and it can be managed.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>