V4L/DVB: ivtvfb : Module load / unload fixes

Check firmware state when loading module & if firmware is not responding exit
with an error. When module is unloaded, only disable the framebuffer & not all
decoder output.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>