iwlwifi: use ieee80211_is_data(fc)

iwl-agn-rs.c already uses this.

Cc: Zhu Yi <yi.zhu@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: ipw3945-devel@lists.sourceforge.net
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>