V4L/DVB (11987): au8522: add support for QAM-64 modulation type

Add support for QAM64 modulation type to the au8522 demod driver.

Signed-off-by: Frank Dischner <phaedrus961@gmail.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>