net: mvneta: add configuration for MBUS windows access protection

This commit adds missing configuration of MBUS windows access protection
in mvneta_conf_mbus_windows function - a dedicated variable for that
purpose remained there unused since v3.8 initial mvneta support. Because
of that the register contents were inherited from the bootloader.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

Fixes: c5aff182

 ("net: mvneta: driver for Marvell Armada 370/XP network
unit")
Signed-off-by: David S. Miller <davem@davemloft.net>