linux-phytec-ti

mac80211: factor out station lookup from ieee80211_build_hdr()

In order to look up the RA station earlier to implement a TX
fastpath, factor out the lookup from ieee80211_build_hdr().
To always have a valid station pointer, also move some of the
checks into the new function.

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