iwlwifi: move debug_level to sysfs/bus/pci/devices

This patch ports the debug_level from sysfs/bus/pci/drivers/iwl4965
to /sys/class/net/wlanX/device/debug_level

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