ath10k: Register shutdown handler

As a part of device shutdown the smmu driver will be
stopped and henceforth any IOVA address translation
will not be done. The wlan driver, being one of the
smmu driver consumer, should stop all the dma related
activity as a part of shutdown, and thereby ensuring
that no dma activity is done once the smmu driver
shuts down.

During the device shutdown, the smmu calls shutdown
for all its consumers in order to indicate them to
stop all their dma activities.

Register the shutdown handler to stop the wlan
driver and avoid any dma operations.

Tested-on: WCN3990 hw1.0 SNOC WLAN.HL.3.1-01040-QCAHLSWMTPLZ-1

Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1593193981-30161-1-git-send-email-pillair@codeaurora.org