iwlwifi: move per driverdebug_level to per device

This patch ports the debug_level to iwl_priv and changes
the format of the debug prints.

Signed-off-by: Ester Kummer <ester.kummer@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>