p54: move statistic timer update routine into a workqueue

This patch moves a good chunk of code from the former statistic update
timer routine into a workqueue, which is kindly provided by mac80211.

Also as a nice side-effect we can lay the foundation for other
essential housekeeping features we want to do in the future.
e.g:
 - drain the (clogged) tx_queue.
 - initiate bursts.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>