mac80211: fix initialization of skb->cb in ieee80211_subif_start_xmit

The change 'mac80211: Fix BUG in pskb_expand_head when transmitting shared skbs'
added a check for copying the skb if it's shared, however the tx info variable
still points at the cb of the old skb

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>