[PATCH] rt2x00: make rt2x00lib_stop_link_tuner() reentrant with link_tuner work

Calling cancel_delayed_work_sync() unconditionally won't hurt
and it will avoid race conditions when another CPU is already
executing link_tuner work.

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