media: saa7146: add cleanup in hexium_attach()

If saa7146_register_device() fails, no cleanup is executed, leading to
memory/resource leaks. To fix this issue, perform necessary cleanup work
before returning the error.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>