qmi_wwan: Set random MAC on devices with buggy fw

Some buggy firmwares export an incorrect MAC address (00:a0:c6:00:00:00). This
makes for example checking devices for random MAC addresses tricky, and you
might end up with multiple network interfaces with the same address.

This patch tries to fix, or at least improve, the situation by setting the MAC
address of devices with this firmware bug to a random address. I tested the
patch with two devices that has this firmware bug (Huawei E398 and E392), and
network traffic worked fine after changing the address.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>