mei: reset flow control on the last client disconnection

The FW resets the flow control for single buffer clients when the last
host client disconnects, also the driver has to follow this policy and
zero the flow control counter in such case.

Cc: <stable@vger.kernel.org> #4.1
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>