wl1251: fix crash on remove due to leftover work item

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.

Cc: stable@vger.kernel.org # v2.6.37+
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>