[media] au0828: Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/media/usb/au0828/au0828-dvb.c:36:5: warning:
 symbol 'preallocate_big_buffers' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/usb/au0828/au0828-dvb.c

@@ -33,7 +33,7 @@
 #include "mxl5007t.h"
 #include "tda18271.h"
 
-int preallocate_big_buffers;
+static int preallocate_big_buffers;
 module_param_named(preallocate_big_buffers, preallocate_big_buffers, int, 0644);
 MODULE_PARM_DESC(preallocate_big_buffers, "Preallocate the larger transfer buffers at module load time");