iwlwifi: check for channel location for 40MHz

for both mixed and pure 40MHz, need to check for valid channel location.
if the specified channel not allow the channel location requested
(ABOVE, BELOW), then reject the Fat channel access

This fixes http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1988



("iwlwifi: checking for 40MHz mode" and "iwlwifi: check control channel
for pure 40MHz" combine with this to address the above bug. -- JWL)

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>