mei: add mei_msg_hdr_init wrapper.

Wrap the mei header boilerplate initialization code in
mei_msg_hdr_init function. On the way remove 'completed'
field from mei_cl_cb structure as this information
is already included in the header and is local to particular
fragment.

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