V4L/DVB (9618): tvaudio: add additional logic to avoid OOPS

This patch checks for volume, bass, treble, set mode and get mode
callbacks before actually enabling the code that would use them.

Instead of aborting the driver for load, this patch will allow it to
load with a reduced number of functionatities.

This prevents OOPS if some board entry is missing a needed callback.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>