V4L/DVB (5021): Cx88xx: Fix lockup on suspend

Suspending with the cx88xx module loaded causes the system to lock up
because the cx88_audio_thread kthread was missing a try_to_freeze()
call, which caused it to go into a tight loop and result in softlockup
when suspending. Fix that.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>