iio: documentation ltc2632_chip_info add num_channels

The documentation for ltc_2632_chip_info missed the desciption for the
num_channels. This trivial patch adds it.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>

Fixes: 9f15a4a0 ("iio: dac: ltc2632: add support for LTC2636 family")
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>