V4L/DVB (5988): Fix OOP on videobuf-dvb when hibernating

Since videobuf_waiton is called with intr=1, it can return -EINTR and therefore
err may be non-zero. This happens when the system goes into the standby state.

Without the BUG() occurring, there's no problem with standby mode while DVB
is being used.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>