iwlwifi: kill iwl_down and s/__iwl_down/iwl_down

iwl_down was just a wrapper around __iwl_down which was called from
one place only. Replace it to direct call to iwl_down. Add lockdep
warning in iwl_down to ensure it was called with the mutex held.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>