[PATCH] bttv: fix DMA setup bug in latest update

This fixes the bug that made the card DMA lots of data to random memory
locations, causing lockups and instant reboots. 

The problem was that the yoffset variable got modified inside the loop,
but the logic in the switch statement was meant to work on the initial
value of the yoffset variable.

(Bug fix extracted from
http://marc.theaimsgroup.com/?l=linux-kernel&m=109532814823565&w=2)
Signed-off-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/media/video/bttv-risc.c

@@ -125,6 +125,7 @@ bttv_risc_planar(struct bttv *btv, struct btcx_riscmem *risc,
 	struct scatterlist *ysg;
 	struct scatterlist *usg;
 	struct scatterlist *vsg;
+	int topfield = (0 == yoffset);
 	int rc;
 
 	/* estimate risc mem: worst case is one write per page border +
@@ -153,13 +154,13 @@ bttv_risc_planar(struct bttv *btv, struct btcx_riscmem *risc,
 			chroma = 1;
 			break;
 		case 1:
-			if (!yoffset)
+			if (topfield)
 				chroma = (line & 1) == 0;
 			else
 				chroma = (line & 1) == 1;
 			break;
 		case 2:
-			if (!yoffset)
+			if (topfield)
 				chroma = (line & 3) == 0;
 			else
 				chroma = (line & 3) == 2;