iio: adc: twl4030_madc: Fix description of twl4030_madc_set_current_generator()

The @chan parameter can be 0 or 1 and not a bit mask. Fix wrong description.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>