V4L/DVB (6287): Fix DMA Scatter/Gather constructor

cx23885 driver were converted to use the newer videobuf support. Unfortunately,
the constructor weren't changed. This causes an oops, since the abstract methods
(implemented as callbacks) aren't defined.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Reviewed-by: Michael Krufky <mkrufky@linuxtv.org>