mac80211: move store skb ack code to its own function

This patch moves the code handling SKBTX_WIFI_STATUS inside the TX path
into an extra function. This allows us to reuse it inside the 802.11 encap
offloading datapath.

Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20191029091304.7330-2-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>