iwlwifi: iwl_poll_{direct_}bit cleanup

The patch merges implementation of iwl_poll_bit() and
iwl_poll_direct_bit() by letting the latter be a special case of
the former.

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