cx231xx: don't return error on success

commit 1871d718a9db649b70f0929d2778dc01bc49b286 upstream.

The cx231xx_set_agc_analog_digital_mux_select() callers
expect it to return 0 or an error. Returning a positive value
makes the first attempt to switch between analog/digital to fail.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>