brcm: Fix Xiaomi Inc Mipad2 nvram/.txt file macaddr

brcm: Fix Xiaomi Inc Mipad2 nvram/.txt file macaddr

Replace the macaddr setting with the special fixed macaddr the kernel's
brcmfmac driver uses to identify that there is no macaddr programmed
into the NIC (NIC programmed makes takes precedence over the .txt one).

This will properly make the kernel generate a random MAC and set
addr_assign_type to NET_ADDR_RANDOM on devices without a MAC programmed
into the NIC.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>