wl1251: fix crash on remove due to leftover work item

commit 4c1bcdb5a3354b250b82a67549f57ac27a3bb85f upstream.

This driver currently leaves elp_work behind when stopping, which
occasionally results in data corruption because work function ends
up accessing freed memory, typical symptoms of this are various
worker_thread crashes. Fix it by cancelling elp_work.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>