rt2x00: Use ieee80211_hw->workqueue again

Remove the rt2x00 singlethreaded workqueue and move
the link tuner and packet filter scheduled work to
the ieee80211_hw->workqueue again.
The only exception is the interface scheduled work
handler which uses the mac80211 interface iterator
under the RTNL lock. This work needs to be handled
on the kernel workqueue to prevent lockdep issues.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>