iwlwifi: remove ieee80211 types from iwl-helpers.h

This patch removes IEEE80211_STYPE_BACK_REQ and
IEEE80211_STYPE_BACK defines from iwl-helpers.h. These are already
defined in include/linux/ieee80211.h

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