iio: adc: xilinx-ams: Fixed missing PS channels

The code forgot to increment num_channels for the PS channel inputs,
resulting in them not being enabled as they should.

Fixes: d5c70627 ("iio: adc: Add Xilinx AMS driver")
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20220127173450.3684318-3-robert.hancock@calian.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>