iwlwifi: check for valid band for channel info

when display channel info in debugfs, always check for valid band
before access the pointer and display information

for 1000 NIC, it only support "bgn" mode, so there is no 5.2GHz channels
available to display.

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>