wil6210: properly initialize discovery_expired_work

Upon driver rmmod, cancel_work_sync() can be invoked on
p2p.discovery_expired_work before this work struct was initialized.
This causes a WARN_ON with newer kernel version.

Add initialization of discovery_expired_work inside wil_vif_init().

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>