wcn36xx: drain pending indicator messages on shutdown

When the interface is shut down, wcn36xx_smd_close() potentially races
against the queue worker. Make sure to cancel the work, and then free all
the remnants in hal_ind_queue manually.

This is again just a theoretical issue, not something that was triggered in
the wild.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>