tvheadend: Disable dvdscan

* when enabled it will try to download
  http://linuxtv.org/hg/dvb-apps/archive/tip.tar.bz2


  from do_configure (bypassing PREMIRROR and bitbake fetcher), but
  what's worse even when it fetches it, it fails in do_install, because
  dvb-scan directory is empty, reading the script called from configure
  3.3-r0/git/support/getmuxlist, I don't know how it could work before,
  because it first moves files to dvb-scan directory and then removes
  them all one by one.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>