[media] dvb_usb_v2: register device even no remote keymap defined

It failed to register device when remote keymap was not set.
Fix it to register device even keymap is NULL. In that case
just skip remote registration.

Driver should set RC_MAP_EMPTY to enable remote in case of
there is remote receiver but default keymap is unknown.

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