cnss2: Clear link down ind flag at the end of shutdown process

During driver unload or idle shutdown, link down may also happen,
which sets the link down ind flag, however once shutdown process
is complete, host driver expects the platform driver to return
false for subsequent is_firmware_down check on the next wifi On.
So clear the link down ind flag at the end of shutdown.

Change-Id: Ice9ace15aac16d7df55b8d9cb433755074e07b1f
Signed-off-by: Mohammed Siddiq <msiddiq@codeaurora.org>