[media] tw68: simplify tw68_buffer_count

The code to calculate the maximum number of buffers allowed in 4 MB
is 1) wrong if PAGE_SIZE != 4096 and 2) unnecessarily complex.

Fix and simplify the code.
Reported-by: NMauro Carvalho Chehab <m.chehab@samsung.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/pci/tw68/tw68-video.c

@@ -361,22 +361,13 @@ int tw68_video_start_dma(struct tw68_dev *dev, struct tw68_buf *buf)
 
 /* ------------------------------------------------------------------ */
 
-/* nr of (tw68-)pages for the given buffer size */
-static int tw68_buffer_pages(int size)
-{
-	size  = PAGE_ALIGN(size);
-	size += PAGE_SIZE; /* for non-page-aligned buffers */
-	size /= 4096;
-	return size;
-}
-
 /* calc max # of buffers from size (must not exceed the 4MB virtual
  * address space per DMA channel) */
 static int tw68_buffer_count(unsigned int size, unsigned int count)
 {
 	unsigned int maxcount;
 
-	maxcount = 1024 / tw68_buffer_pages(size);
+	maxcount = (4 * 1024 * 1024) / roundup(size, PAGE_SIZE);
 	if (count > maxcount)
 		count = maxcount;
 	return count;