rndis_host: Set valid random MAC on buggy devices

Some devices of the same type all export the same, random MAC address. This
behavior has been seen on the ZTE MF910, MF823 and MF831, and there are
probably more devices out there. Fix this by generating a valid random MAC
address if we read a random MAC from device.

Also, changed the memcpy() to ether_addr_copy(), as pointed out by
checkpatch.

Suggested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>