[media] tw686x: silent -Wformat-security warning

Using sprintf() with a non-literal string makes some compiler complain
when building with -Wformat-security (eg. clang reports "format string
is not a string literal (potentially insecure)").

Here sprintf() format parameter is indirectly a literal string so there
is no security issue.  Nevertheless adding a "%s" format string to
silent the warning helps to detect real bugs in the kernel.
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/tw686x/tw686x-core.c

@@ -74,7 +74,7 @@ static const char *dma_mode_name(unsigned int mode)
 
 static int tw686x_dma_mode_get(char *buffer, struct kernel_param *kp)
 {
-	return sprintf(buffer, dma_mode_name(dma_mode));
+	return sprintf(buffer, "%s", dma_mode_name(dma_mode));
 }
 
 static int tw686x_dma_mode_set(const char *val, struct kernel_param *kp)