wifi_statistics: count fcsfail packets for nullmac

If the fcsfail packets are broken beyond repair, count them for the null
mac. This prevents the hash from getting filled with useless station
entries. If the mac can still be found in the hash, assume that the
packet is not so broken and count it for this station.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
wifi_statistics: add compat code for eth_zero_addr()

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>