mei: reduce flow control only for completed messages

This fixes bug when only first chunk of a large message split
by hbuf_max_len is written to the hardware.
All the consequent chunks will not get a new credit.

A regression introduced by the commit
0ef319c93cebff9f82bdd0cdbb298f2dd00acda8
mei: streamline write complete flow function

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