iwlwifi: clean up in setup/cancel deferred work

This patch makes some clean up in setup/cancel_deferred_work.
iwl_setup_deferred_work does the work that is common to 4965
and 5000, then it calls to HW specific handlers.

This patch also removes uneeded work_struct from iwl_priv.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>