media: pci: cx23885: replace BUG with error return

It was completely unnecessary to use BUG in buffer_prepare().
Just replace it with an error return. This also fixes a smatch warning:

drivers/media/pci/cx23885/cx23885-video.c:422 buffer_prepare() error: uninitialized symbol 'ret'.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/pci/cx23885/cx23885-video.c

@@ -413,7 +413,7 @@ static int buffer_prepare(struct vb2_buffer *vb)
 				dev->height >> 1);
 		break;
 	default:
-		BUG();
+		return -EINVAL; /* should not happen */
 	}
 	dprintk(2, "[%p/%d] buffer_init - %dx%d %dbpp 0x%08x - dma=0x%08lx\n",
 		buf, buf->vb.vb2_buf.index,