staging iio adt7410: make 16bit mode default

In 13bit mode the lower three bits of the adc value contain
flags. The driver does not use these flags at all, so make 16bit
mode the default.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>