iio: at91-sama5d2_adc: split at91_adc_current_chan_is_touch() helper

This change moves the logic to check if the current channel is the
touchscreen channel to a separate helper.
This reduces some code duplication, but the main intent is to re-use this
in the next patches.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>