DVB: isl6421: don't reference freed memory

After freeing a block there should be no reference to this block.

(cherry picked from commit 09d48954)
Signed-off-by: Thomas Viehweger <Thomas.Viehweger@marconi.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>