mei: make mei_cl_set_disconnected static

mei_cl_set_disconnected is used only in client.c,
so make it local to the file and mark static.

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>