V4L/DVB: DVB: Export dvb_ringbuffer_flush() again

Add EXPORT_SYMBOL(dvb_ringbuffer_flush) again,
replace my_dvb_ringbuffer_flush() in ngene driver.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>