cnss2: Clear flag CNSS_DRIVER_UNLOADING after unregister driver finished

Flag CNSS_DRIVER_UNLOADING is set when unregister wlan started, but it's
cleared in cnss_shutdown, which is also used in other call flow. Clear this
flag after unregister finished.

Change-Id: I0b5d63e5570c2604ff66d7852d7c7ca1c008a475
Signed-off-by: Kai Liu <kaliu@codeaurora.org>