[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>