mac80211: remove skb header offset mangling in ieee80211_build_hdr

Since the code only touches the MAC headers, the offsets to the
network/transport headers remain the same throughout this function.
Remove pointless pieces of code that try to 'preserve' them.

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