mei: use structured buffer for extra write buffer

The structure of the message is static so we don't have
to use and cast the buffer. We can also drop extra_write_index
variable as this information can be extracted directly
from the message header

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/misc/mei/init.c b/drivers/misc/mei/init.c
index 02784af..49600d6 100644
--- a/drivers/misc/mei/init.c
+++ b/drivers/misc/mei/init.c
@@ -288,7 +288,7 @@
 		mei_me_cl_unlink(dev, &dev->iamthif_cl);
 
 		mei_amthif_reset_params(dev);
-		dev->extra_write_index = 0;
+		memset(&dev->wr_ext_msg, 0, sizeof(dev->wr_ext_msg));
 	}
 
 	dev->me_clients_num = 0;