wimax/i2400m: generate fake source MAC address with random_ether_addr()

The WiMAX i2400m driver needs to generate a fake source MAC address to
fake an ethernet header (for destination, the card's MAC is
used). This is the source of the packet, which is the basestation it
came from. The basestation's mac address is not usable for this, as it
uses its own namespace and it is not always available.

Currently the fake source MAC address was being set to all zeros,
which was causing trouble with bridging.

Use random_ether_addr() to generate a proper one that creates no
trouble.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>