iio: adc: cpcap-adc: kernel-doc fix - that should be _ in structure name

Fixes a W=1 warning.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210314164655.408461-6-jic23@kernel.org