icnss: Do not send uevent when driver is unloading

If modem crashed during wlan driver unloading, icnss driver
will call driver uevent callback to send FW down uevent when
it receives BEFORE_SHUTDOWN notification. If wlan driver is
de-initialized just before the callback is called, kernel will
panic as driver's context is freed. This can be avoid by not
sending uevent when wlan host driver is unloading. Instead,
icnss driver will provide an API to host driver to check if
WLAN FW is down or not.

CRs-Fixed: 2161425
Change-Id: I569fd85366522606ececeda74df85c51b9b2fc28
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>