iwlwifi: remove the statistics work

This patch does 3 things

1) It removes the statistics work. The request statistics command is
sent in ASYNC mode in this flow, the mutex is uneeded, so the request
statistics function can't go to sleep. No need for a workqueue anymore.

2) iwl4965_send_statistics_request has been renamed to
iwl_send_statistics_request and moved to iwl-core.c

3) A request for statistics is sent in alive_notify, the makes the uCode
sends statistics notification periodically starting from association.

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