iio: adc: mcp3422: Changing initial channel

Initial channel should be the first available channel on
all configurations, so changing to channel 0 available on
all supported chips.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reported-by: Maarten Brock <m.brock@vanmierlo.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>