mei: fix memory leak of pending write cb objects

Write callbacks are released on the write completed path but
when file handler is closed before the writes are
completed those are left dangling on write and write_waiting queues.

We add mei_io_list_free function to perform this task

Also move static functions to client.c form client.h

Cc: stable <stable@vger.kernel.org> # 3.11+
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>