V4L/DVB (6192): Ensure start_dma() is capable of starting dma on port VIDB

start_dma() would fail to start dma if a device used VIDB (portb).

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