cnss2: Fix a few issues during platform reboot or shutdown

Make unregister driver work unkillable to avoid WLAN host driver
handling issues after it is killed by platform reboot or shutdown.
Also add proper handlings and checks for platform reboot or shutdown
to avoid unnecessary asserts for calibration cases.

Change-Id: Ide137dc351ca48ef23573abb325db011b8460f54
Signed-off-by: Yue Ma <yuem@codeaurora.org>