[media] dvb_usb_v2: try to remove all adapters on exit

It did not removed adapter on error case as .num_adapters_initialized
was set after success adapter initialization. It should be safe to
try remove all possible adapters in any-case.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>