mac80211: Fix dropping broadcast packets in 802.11 encap

Broadcast pkts like arp are getting dropped in 'ieee80211_8023_xmit'.
Fix this by replacing is_valid_ether_addr api with is_zero_ether_addr.

Fixes: 50ff477a ("mac80211: add 802.11 encapsulation offloading support")
Signed-off-by: Seevalamuthu Mariappan <seevalam@codeaurora.org>
Link: https://lore.kernel.org/r/1591697754-4975-1-git-send-email-seevalam@codeaurora.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>