ath10k: utilize random MAC address if none is provided

Since commit 9d580466

 ("ath10k: retrieve MAC address from system
firmware if provided"), we support pulling our MAC address from system
firmware. Along with this, we should admit that it's possible for system
firmware to be screwed up as well, omitting the MAC address there.

Provide a fallback, such that we choose a randomly-generated MAC
address, with the local assignment bit set.

This is not a preferred mode of operation and really should only serve
the purpose of keeping the system chugging along while someone figures
out what's wrong with the system. Grepping through other wireless
drivers shows very similar fallback logic though, so why not?

  git grep eth_random_addr drivers/net/wireless/

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>