V4L/DVB (8067): cx18: Fix firmware load for case when digital capture happens first

This is a fix for the case when a digital capture from dvr0 happens first after
modprobe, before access to any cx18 v4l2 device nodes.  The initial dvb feed
start has been changed to load the firmware if not already loaded.  Also fixed a
use counter to correct dvb feed accounting if starting the transport DMA fails.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>