iwlwifi: refactor ieee80211_get_qos_ctrl

This patch refactors ieee80211_get_qos_ctrl function and its usage
in iwlwifi drivers. Function is moved as inline into iwl-helpers.h.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>