staging/mei: don't use read buffer for writing

In mei_irq_thread_read_bus_message we reused mei_hdr
allocated on read buffer to write the stop message.
There is no bug associated with this but for code
clarity we use write buffer also for message header.

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