wifi: brcmfmac: Replace default (not configured) MAC with a random MAC

On some boards there is no eeprom to hold the nvram, in this case instead
a board specific nvram is loaded from /lib/firmware. On most boards the
macaddr=... setting in the /lib/firmware nvram file is ignored because
the wifi/bt chip has a unique MAC programmed into the chip itself.

But in some cases the actual MAC from the /lib/firmware nvram file gets
used, leading to MAC conflicts.

The MAC addresses in the troublesome nvram files seem to all come from
the same nvram file template, so we can detect this by checking for
the template nvram file MAC.

Detect that the default MAC address is being used and replace it
with a random MAC address to avoid MAC address conflicts.

Note that udev will detect this is a random MAC based on
/sys/class/net/wlan0/addr_assign_type and then replace this with
a MAC based on hashing the netdev-name + the machine-id. So that
the MAC address is both guaranteed to be unique per machine while
it is still the same/persistent at each boot (assuming the
default Link.MACAddressPolicy=persistent udev setting).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220708133712.102179-2-hdegoede@redhat.com