mac80211: explicitly check skb->len

ieee80211_get_hdrlen_from_skb internally checks the skb is long enough to
hold the full ieee80211_hdr, else it returns zero.  Use ieee80211_hdrlen
which always returns the hdrlen and check the remaining room in the
skb explicitly when removing encryption headers or the qos control field.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>