iio: chemical: atlas-sensor: add helper function atlas_buffer_num_channels()

Add helper function to detect the number of channels to output
in trigger handler. This is based on IIO_TIMESTAMP channel being the
delimiter between input and output channels.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>