mei: do not override a client writing state when buffering

when we buffer write request we should not switch to
MEI_WRITING since this will override MEI_WRITE_COMPLETE
state of preceding write

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