There are some races in the hardware that can possibly lead to
a bus lockup later during a restart when we manage to kill the
firmware at a bad time (while it's accessing the bus).

To work around this, add support for a new handshake between
firmware and driver to ensure that the firmware is in a well-
known state before we kill it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201209231352.7756fcc9865c.I13de65e0ffcb4186dd4c1a465f66df2e98c9a947@changeidSigned-off-by: Luca Coelho <luciano.coelho@intel.com>