media: bttv: Fixed oops error when capturing at yuv410p

When capturing at yuv410p, sg_next was called too many times when chroma is
false, eventually returning NULL. This patch does fix this for my hardware.
Signed-off-by: Samuel Williams <sam8641@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/pci/bt8xx/bttv-risc.c

@@ -189,6 +189,12 @@ bttv_risc_planar(struct bttv *btv, struct btcx_riscmem *risc,
 				yoffset -= sg_dma_len(ysg);
 				ysg = sg_next(ysg);
 			}
+
+			/* calculate max number of bytes we can write */
+			ylen = todo;
+			if (yoffset + ylen > sg_dma_len(ysg))
+				ylen = sg_dma_len(ysg) - yoffset;
+			if (chroma) {
 				while (uoffset && uoffset >= sg_dma_len(usg)) {
 					uoffset -= sg_dma_len(usg);
 					usg = sg_next(usg);
@@ -198,11 +204,6 @@ bttv_risc_planar(struct bttv *btv, struct btcx_riscmem *risc,
 					vsg = sg_next(vsg);
 				}
 
-			/* calculate max number of bytes we can write */
-			ylen = todo;
-			if (yoffset + ylen > sg_dma_len(ysg))
-				ylen = sg_dma_len(ysg) - yoffset;
-			if (chroma) {
 				if (uoffset + (ylen>>hshift) > sg_dma_len(usg))
 					ylen = (sg_dma_len(usg) - uoffset) << hshift;
 				if (voffset + (ylen>>hshift) > sg_dma_len(vsg))