iwlagn: kill iwlagn_setup_deferred_work

Since iwlagn_setup_deferred_work is always called, fold it into
iwl_setup_deferred_work. BT related works are setup by the new
bt_setup_deferred_work lib_ops.

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