Commit "iwlwifi: make debug level more user friendly" cleaned up the
debug level handling. In doing so it created a single global debug
level for all devices. Some setups do consits of more that one iwlwifi
device and in these setups there is a requirement that debug levels
should be unique per device.

We now re-introduce the per device debugging while maintaining the
cleanup effort of the previous patch.

The maintain the global debug level and now introduce a per-device debug
level that will be used if it (the per-device debug level) is set. The
per-device debug level can be controlled via the debug_level sysfs file
while the global debug level is controlled by the debug module parameter.
Signed-off-by: NReinette Chatre <reinette.chatre@intel.com>
Acked-by: NTomas Winkler <tomas.winkler@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>