iwlwifi: remove per-device debug level

There's no need for the per-device debug
level that we expose in debugfs since the
module parameter is writable in sysfs.

At the same time, simplify code by changing
    iwl_get_debug_level(shrd) & IWL_DL_ISR)
to
    iwl_have_debug_level(IWL_DL_ISR)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>