iwlwifi: check for valid ethernet address provided by OEM

In 9000 family products we added an option to let the OEM fuse the
mac address via registers. If these registers are zeroed we use the OTP
address instead. Make sure that the address provided by the OEM is valid
and, if not, fall back to the OTP address as well.

Fixes: commit 17c867bf ("iwlwifi: add support for getting HW address from CSR")
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>