mac80211: move QOS control helpers into ieee80211.h

Also remove the WLAN_IS_QOS_DATA inline after removing the last
two users.  This starts moving away from using rx->fc to using
the header frame_control directly.

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