[media] dvb-bt8xx: use goto based exception handling

Repeating the same cleanup code in each error handling path makes life
unnecessarily difficult for reviewers, who much check each instance of
the same copy+pasted code separately.  A "goto" to the end of the
function is more maintainable and conveys the intent more clearly.

While we're touching this code, also lift some assignments from "if"
conditionals for simplicity.

No functional change intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>