iio: adc: mcp3422: Checking for error on probe

Some part of the configuration are not touched after the probe
and if something goes wrong on writing the initial one,
the chip will misbehave.
Adding an error checking ensures that the inital configuration will
be written correctly. Moreover ensures that a sensible configuration
will be saved in driver data and used subsequently as intended.

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>