video: fbdev: arcfb: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 115017 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Cc: Jaya Kumar <[e-mail]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>

drivers/video/fbdev/arcfb.c

···
 			schedule();
 			finish_wait(&arcfb_waitq, &wait);
 		}
+		/* fall through */
+
 		case FBIO_GETCONTROL2:
 		{
 			unsigned char ctl2;