mac80211: reject zero MAC address in add station

This came up in fuzz testing, and really we don't consider
all-zeroes to be a valid MAC address in most places, so
also reject it here to avoid confusion later on.

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Link: https://lore.kernel.org/r/1563959770-21570-1-git-send-email-periyasa@codeaurora.org


[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>