use VSB IF frequency ( 44 / 5.38 MHz ) if qam_if is invalid or unspecified
Acked-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>