staging/mei: propagate error codes up in the write flow

Change mei_write_message's return type from bool to int
to enable propagation of the error code up to caller functions.
The function now returns -EIO when low level write fails and 0
on success.
A similar change is done in intermediate caller functions:
mei_send_flow_control, mei_connect, and mei_disconnect

This makes code more alike to typical Linux kernel error
handling.

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