V4L/DVB (13410): firedtv: remove an unnecessary function argument

All read transactions initiated by firedtv are only quadlet-sized, hence
the backend->read call can be simplified a little.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>