[media] dvb_usb_v2: fix error handling for .tuner_attach()

fe was not set NULL after it was destroyed in tuner attach fail
error case. Due to that it was destroyed again and Kernel oopsed.

Reported-by: Oliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>