mei: allow clients on bus to communicate in remove callback

Introduce new intermediate state to allow the clients on the bus
to communicate with the firmware from the remove handler.
This is to enable to perform a clean shutdown.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20210206144325.25682-2-tomas.winkler@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>