mac80211: add ieee80211_tx_prepare_skb() helper function

This can be used by a driver to prepare skbs for transmission, which were
obtained via functions such as ieee80211_probereq_get or
ieee80211_nullfunc_get.

This is useful for drivers that want to send those frames directly, but
need rate control information to be prepared first.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>