mei: add mei_cl_write function

consolidate write code to a specific me client in mei_cl_write function
the function is called from mei device write handler and from
mei_cl bus send function

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>