V4L/DVB (3801): Cx88-tvaudio: replace 'if' with 'switch..case'

This patch cleans up a potential mess that has yet to occur in the
card-specific part of cx88-tvaudio.c that sets sound input from external adc.
It may be a good idea to move this setting into cx88-cards.c in the future.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>