wcnss: Avoid crash during shutdown in SSR

Wlan driver should not read registers from wcnss or
try to communicate with the chip when it is undergoing
a subsystem-restart. this flag will be used by the wlan
driver to protect register access and other prohibited
operations.

CRs-Fixed: 834133
Change-Id: I638ee9e6cbc25da77ca931191718cec3a7aa87d6
Signed-off-by: Hardik Kantilal Patel <hkpatel@codeaurora.org>