[media] bt8xx: fix memory leak

If dvb_attach() fails then we were just printing an error message and
exiting but the memory allocated to state was not released.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>