iwlwifi: make debug level more user friendly

* Deprecate the "debug50" module parameter used to obtain
  5000 series and up debugging. Replace it with "debug" module
  parameter to match with original driver and be consistent
  between them. The "debug50" module parameter can still be used,
  except that the module parameter is not writable in keeping
  with its previous state. We currently just mark it as "deprecated"
  and do not have it in the feature-removal-schedule. Some more
  cleanup of module parameters needs to be done and can then be
  entered together.

* Only make "debug" module parameters visible if the driver
  is compiled with CONFIG_IWLWIFI_DEBUG. This will eliminate
  a lot of confusion where users think they have set debug flags
  but yet cannot see any debug output.

* Make module parameters writable. This eliminates the need for the
  "debug_level" sysfs file, which can now also be deprecated and
  added to feature-removal-schedule. This file is in significant
  use though with many iwlwifi documents and text referring users
  to it. We can thus not take its removal lightly and keep it around.

With iwlcore shared between iwlagn and iwl3945 we really do not need
debug module parameters for each but can instead have one debug
module parameter for the iwlcore module. The same issue is here as
with the sysfs file - a lot of iwlwifi documentation and text (like
bug reports) rely on iwlagn and iwl3945 having this module parameter,
so changing this to a module parameter of iwlcore will have significant
impact and we do not do this for that reason.

One consequence of this patch is that if a user is running a system
with both 3945 and later hardware then the setting of the one module
parameter will affect the value of the other. The likelihood of this
seems low - and even if this setup is present it does not seem like an
issue for both modules to run with the same debug level.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>