V4L/DVB: cx88: convert core->tvaudio into an enum

Using an enum and removing the default case from switch statements accessing
the value enables the compiler to emit a warning (enabled with -Wall) when an
audio mode is not handled.

This highlights an omission in the function cx88_dsp_detect_stereo_sap()
(in cx88-dsp.c) not handling WW_EIAJ and WW_M.

Signed-off-by: Lawrence Rust <lawrence@softsystem.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>