mei: rx flow control counter

Use RX flow control counter in the host client structure to
track the number of simultaneous outstanding reads.
This eliminates search in queues and makes ground for
enabling for parallel read.

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>