[media] cx231xx: don't return error on success

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.

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>